The reverse crunch is a variation in which the legs and hips move while the trunk is anchored.
 
Lie on the floor and hold a stable rack or weight behind your head. Keeping the knees bent, lift and curl the pelvis up to create the same basic motion you would in a crunch, but with the trunk anchored instead of the legs. Avoid kicking or swinging the legs to create momentum—create the movement by contracting the abs. A foam roller or similar can be held between the ankles and the hamstrings to help eliminate any leg movement. 
 
Bring the feet back as close to the floor as possible without allowing them to touch between reps. Maintain ab tension throughout the motion—never let the lower back hyperextend in the bottom.
 
Purpose
The reverse crunch is simply a variation of the crunch that provides another way to focus on abdominal strength or stamina.
 
Programming
The reverse crunch should be done for sets of 10-30+ reps.
 
Variations
The reverse crunch can be performed on a glute-ham bench or lying on a swiss ball or similar to increase range of motion into lumbar extension.