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 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, keeping the abs crunched to maintain a hollow position and continuous tension.
 
Notes
The jack knife can be considered an easier variation of the V-up and used as an introductory exercise to build toward it.
 
Purpose
The jack knife provides some additional challenge and variation to the sit-up, trains both the abs and hip flexors, and is good to maintain continuous tension in the abs.  
 
Programming
The jack knife should be performed for sets of 10-30+ and will generally be more of a stamina exercise with higher reps.
 
Variations
The jack knife can be done with a pause at the top and/or bottom, with rotation of the trunk and with a single knee raise. Weight can be added by laying a plate on the shins.