Remember that DYNAMICS can occur in rhythms. People will naturally “lay back” or “push” at certain places in a song or a repeating rhythmic groove.  Most rhythmic dynamics happen naturally (without notice rather than intentional) and often occurs because a drummer is “leaning” a certain way (or even because they are not yet experienced enough to play with consistency). 

For example, a punk rock drummer will tend to play with more energy than calculated thought, and as a result some of the drum hits may be ahead of the exact place they are intended for.  This explains why punk rock snare drums are pushing the beat more often than big rock ballad snare drums.

If a certain rhythmic note is important in a style of music, a drummer may unconsciously emphasize that beat by playing it harder, and unless they begin to play the note earlier than they play other notes the extra effort required to play harder may actually cause the note to be hit slightly later, giving it a laid back feeling that can actually make a beat feel heavier.  Remember, the drummer may rush to hit that all-important note.

The end result is that naturally performed drum parts WILL contain certain internal dynamics rather than be precise and exact.  Furthermore there is important emotion expressed in rhythmic dynamics, which is why music made with drum machines that play each element EXACTLY on the beat is often considered mechanical and “unnatural”.  To compensate, composers often will add repeating loops of live drumming to their machine drums in order to add the missing rhythmic dynamics.

Drum loops can be tricky.  A drum loop is a repeated drum phrase (usually one or two bars in length).  Many drum loops contain rhythmic dynamics, and certain drum hits will be slightly off time.  When using several drum loops, it is possible to create moments when drum parts are slightly off time in DIFFERENT directions.  This can go beyond a rhythmic “smudge” and sound (or feel) like a mistake. People who use multiple drum loops often shift their relative positions to minimize blatant problems.  Then again, many people just don’t care, and simply throw things together until it kind of sounds cool as their compositional process.