I am a personal trainer, and many of my clients beg for this little exercise!! It's effective!!!
Sprinklers:
Begin in a regular crunch position, feet on the floor, knees bent. Complete a full crossover (left shoulder to right leg), then come up on a diagonal to the right leg (simply, it's a smaller crossover crunch or "twist" to the right leg), then come up center, then diagonal right shoulder to left leg, then full crossover right shoulder to left leg. That's one rep. As your clients progress in strength, have them "rewind" to the beginning. That's one rep! As they become even stronger, have them "rewind", but have them stay at the top of the crunch for the entire "rewind". That's one rep. Vary intensity by having legs in the air, on the bosu or ball, with a band for added resistance...
Have fun!!!