Pause Jack Knife
The pause jack knife is a variation of the jack knife sit-up that adds a pause at the bottom and/or top of each rep.
Lie on the floor with the legs extended straight and your hands at the sides or behind your head where you would place them for a sit-up or crunch. Simultaneously perform a sit-up with the trunk and bend your knees to lift them up to bring the knees and elbows together. Return to an extended body position without allowing the heels to touch the floor in between reps. The pauses and holds can be added at the top, bottom or both for 1-5 seconds or more.
The rotating jack knife can be done with rotation and with or without pauses at the bottom and/or top, and transformed into a V-up
by keeping the legs extended and the hands extended overhead.