Doubles are extremely useful, and not just in a roll context. Isolated double strokes can be played around the drumset to produce musical effects or to give you a little extra time to prepare for your next drum or cymbal hit. In addition to those obvious applications, being able to play a good-quality double-stroke roll will likely improve your drumming in ways you wouldn’t expect.
Define the Diddle
Proper double strokes (or “diddles”) aren’t simply bounced off the head. Nor are they played with two individual wrist strokes. At slow to medium tempos, doubles should be played as flowing free strokes (aka full or legato strokes) where the first stroke is played mainly with the wrist and the second is played mainly with the fingers (the “alley-oop” technique).