Drumming is the result of Reich's African studies and can be seen as a transitional point between early phase music and tape experiments and later, more symphonic (dammit, more sophisticated) fare.
In the early tape-loop pieces like "Come Out" and "Its gonna rain", two tape loops of slightly different length are played against each other so that they go in and out of phase with each other. To the listener, the effect is like gradually adding distortion until, at maximum phase difference, the taped phrase is hardly recognisable.
After taped speech patterns, Reich made similar phase pieces for organs, violins and pianos, just to see if it could be done, I suppose.
Drumming begins by applying his patent phase technique to drum patterns, and for twenty minutes we listen, hypnotised, as the phasing plays out in the familiar its-all-predetermined way. Then, rather than winding itself up as previous phase experiments have done, OTHER INSTRUMENTS JOIN IN. For Reich to do this at this point has great impact as he breaks away from purist minimalism.
We hear, for the first time, Reich's trademark percussion/melody/solfege vocal mix. The increasing complexity is like a beautifully flourishing flower after the dry seed of his minimalistic phase music, and it paves the way for the later riches of The Desert Music, The Cave, Different Trains and so on.