To fully develop one's pocket in time keeping, it's important that we first realize that the beat/pulse of the music is actually very wide. There's not just one minuscule point in time where a pulse exists, such as the quarter-note in 4/4 or Common Time, but rather it encompasses numerous milliseconds. To better understand this, look at how a Flam works. The grace note occurs just slightly ahead of the main note, thus making a fuller, thicker sound. We still feel and hear the Flam occurring within the given beat/pulse, even though the left and right hand strokes do not strike the playing surface at the same exact point in time.
The Concept — Back, Middle, or Top of the Beat

Think of each pulse in the music like a canoe or boat. The canoe exists over a space in the water, which represents the music; the current and/or direction that the boat flows in represents the time. The speed of the boat floating down the river is the tempo. Note that the boat doesn't just touch one small point in the water; it exists over a substantial portion of the water's surface.
If the boat represents one pulse in the music, let's say in this case a quarter-note, there are actually many different areas we can place ourselves, and yet still remain in the boat (pulse). When everyone in the band sits in the middle of the boat, there's perfect balance, and that's a good thing! But what if we all move to the front of the boat, with no one in the center or back of the boat? The vessel will eventually flip over ... if enough people are up front or on top. The same goes if we all go to the back of the boat, the vessel will capsize. So to maintain a good balance in the canoe, we need to spread out evenly or stay closer to the middle of the boat. Some of us can move to the front of the boat, while others remain in the center or back of the vessel, for example.
I don't want to over analyze this, but it bares mentioning that we all seem to naturally gravitate to the side of the boat that makes the journey feel the most comfortable; removing the feeling of capsizing the vessel. Music is the same way. We position ourselves within the pulse so as to create a balanced feel ... while still giving of sense of "driving on top", "sitting down the middle" or "laying back".
Feel the Music — Stay in the Boat
To me, different styles and genres of music have different relationships as to this placement. In uptempo Cuban music, for example, I tend to hear the drums/percussion playing more on top, and the bass landing in the middle to top. The other instruments fall somewhere on or between the middle and front of the pulse. This is one of the few genres that always feels like the boat is going to tip over ... to my ears ... because everyone is "driving on top" ... but they maintain the balance. If they were to flip the boat over, it would be realized in musical terms as rushing or just falling apart.
In the bulk of Pop music, I hear the drums laying back and the bass is more down the middle. The guitar, keys and vocals are on top to down the middle. Again, everyone is sitting in the boat in such a way as to bring balance to the music ... thus creating it's own vibe.
So how does all of this apply to grooving or having a pocket in our time keeping? Well, let's take this concept of "balance" and adapt it for the drumkit by looking at the placement of the three commonly used time-keeping voices; the Kick, Snare and Hi-Hat.
If we take all three voices (Kick, Snare, Hi-Hat) and put them all on top of the beat/pulse (in the front-end of the boat), it doesn't automatically mean that we're rushing (capsizing the boat), but it does mean that we run the risk of doing so. The same could be said if we place everything in the back of the boat. Anytime you place yourself on the far edge of the boat (beat/pulse), you are getting into a dangerous area. Why? Because it's hard to remain balanced, not flipping the boat, when you are on the extreme edge. As time keepers, getting on the far edge, whether it be in the front or back of the boat (pulse), we run the risk of going over ... that is flipping the beat over by rushing or dragging ... which wrecks the time flow.
Practical Playing Applications
When I'm trying to invoke a mood, such as "laying back" or "driving on top", I typically let it come from the Snare more than anything else. The reason that I do this is that I want to have (actually need to have) something grounded within the kit. For me, it's my Kick drum. I put it right down the center of the beat ... or at least try to ... regardless of the style. This is just a general rule, but not always the case ... it depends on the music or song. I also keep the Hi-Hat or Ride pretty much down the center of the beat (although when playing jazz, for example, I will drive the band, on top, with the Ride cymbal). This allows me to position my Snare backbeats in the correct section of the pulse for the feel I'm wanting to convey. To my ears, the Snare seems to be the dominant factor in establishing a feel in most genres where the drummer is playing backbeats.
I demonstrate this in my clinics (see audio examples below) by playing along to a pre-recorded track. Even though the rest of the instruments don't move, I'm able to create a variety of feelings by laying back on the backbeats, playing dead on, or playing on top. I'm not trying to say that you don't or can't move other parts of the kit within the pulse, I'm just suggesting that the backbeat placement can make a huge difference ... without having a need for the other instruments in the band to move or change. If you start laying back on the Kick drum, your bass player is going to more than likely need to move with the Kick. Slight placement differences between the Kick and bass can create some interesting results ... like making the bottom-end sound VERY big.
Now ... there's a REAL danger in getting all analytical with this stuff. You can really start messing with your mind, destroy the groove, and get away from what it's all really supposed to be about ... the music.
I once read an article in which session great J.R. Robinson stated that he doesn't like thinking along the lines I've discussed here. J.R. just plays ... and plays very well. Perhaps he does a lot of this but just doesn't think about it. Anyway, for those of us, like myself, who do not have "the gift", being able to analyze all of this gives me a way to approach my technique for a given song ... and also provide a way to communicate the concept in an educational manner.
All of this talk is quite controversial; some agree, some disagree. You have to come up with your own opinion, and do what works for you.
I suggest that you (everyone) should record themselves and listen back to you playing. It's THE way to learn and understand this great mystery of feel.
Performance Demonstration
The three tracks below demonstrate how to play "down the middle", "on the backside", and "on top" of the beat. I used the first cut, "Straight Eighths", from Dave Weckl's Ultimate Play-Along CD - Volume 1 for my backing track.
Notice that I kept my parts very simple so you can listen and feel the differences as you compare the three variations. I focused on the placement of my Snare backbeats to achieve the feel; keeping the Kick and Hi-Hat straight down the middle to line-up with the pre-recorded tracks. In a live situation, you would want to adjust your part(s) to fit with the rest of the band. These examples are merely an experiment to help you better understand how the groove works. I'm not suggesting that the Snare drum is the only way to create these various feels, but we need to start somewhere, so choosing one part of the kit (ie. Snare) to alter makes it easier to hear and learn.
AUDIO EXAMPLES
Playing on the BACK
Playing in the MIDDLE
Playing on the TOP
Keep in mind that with the above examples, my approach and performance are for educational purposes only, in an effort to help you the listener actually hear what many still view as a mystery.
If I were to play this particular tune in a live setting, I would have chosen to place the Snare drum somewhere between what you heard in the Middle and Back of the beat examples, which to my ears is the best pocket for this tune. My conclusion is largely based on how the other instrumentalists are playing their parts, as well as the nature of the music itself. The bottom line ... make the music feel good!
It's a beautiful thing really. Being able to have so much flexibility just with the backbeat. Now you can HEAR the differences, and see how the placement of the backbeat AND the consistency of that backbeat ... is very important when it comes to establishing a pocket and maintaining the groove.