Lie on the floor with the legs extended straight and your hands at your head where you would place them for a sit-up or crunch.
 
Simultaneously perform a rotating sit-up with the trunk and bend your knees to lift them both up equally, bringing the elbow of one side to meet the knee of the other.
 
Return to an extended body position without allowing the heels to touch the floor in between reps, keeping the abs crunched to maintain a hollow position, then rotate in the other direction for the next rep.
 
Purpose
The rotating jack knife provides some additional challenge and variation to the sit-up, trains both the abs and hip flexors, trains trunk rotation, and is good to maintain continuous tension in the abs.  
 
Programming
The rotating jack knife should be performed for sets of 10-30+ and will generally be more of a stamina exercise with higher reps.
 
Variations
The rotating jack knife can be done with a pause at the top and/or bottom, and with a single knee raise.