This is a great chin-up variation because it functions as both a back exercise and a core exercise.
Raise your legs until they're parallel to the floor and don't allow them to drop for the duration of the set. Once you've established good body position, perform chin-ups as normal, pulling your upper chest to the bar on each rep. If you can, try to angle your torso so you're leaning back slightly to further emphasize the lats.
You can use any grip you'd like, but it's best to vary it periodically. Having your legs raised in front of you keeps you honest because you can't use momentum from the lower body to propel yourself up.
Make It Easier
You may initially lack the core strength and/or hamstring flexibility to do this with straight legs, in which case you can start with your legs bent and work on straightening them over time.
Make It Harder
The position of the legs makes it difficult to add weight with a dip belt, so if you're looking to make it harder, you can slow down the tempo, add a weight vest, put a small dumbbell between your feet, or use light ankles weights (absolutely brutal)!