The jack knife is an advanced variation of the sit-up involving the raising of both the legs and trunk; it can also be thought of as a scaled variation of the V-up.
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 meet your elbows at the top of the sit-up. Your balance should be such that you can pause when the knees and elbows meet and hold this position. Return to an extended body position without allowing the heels to touch the floor in between reps.
The jack knife can be considered an easier variation of the V-up
The jack knife provides some additional challenge and variation to the sit-up, and trains both the abs and hip flexors. It can also be used as a way to scale the V-up to work toward them.
The jack knife should be performed for sets of 10-30+ and will generally be more of a stamina exercise with higher reps.
The jack knife can be transformed into a V-up
by keeping the legs extended and the hands extended overhead.