The L-sit is a simple isometric hip flexor and ab exercise.
 
 
Execution
 
Support yourself with straight arms on a pair of parallettes or dumbbells and lift your straight legs until horizontal, then hold this position for the prescribed duration.
 
 
Purpose
 
The L-sit trains general trunk stability and hip flexor strength in a simple and easily controlled exercise.  
 
 
Programming
 
The L-sit can be done with other trunk and accessory work at the end of the training session. 3-5 sets is plenty, and holds can be done for maximal time (short of actual failure), with predetermined rest-work intervals, or with the goal to accumulate a certain total hold time resting as needed.
 
 
Variations
 
The L-sit can be scaled by bending the knees—the more bend, the easier the hold will be.  L-sits can also be done hanging from a bar.