You may be asking what leg raises have to do with your abs. Well, the core is required in a lot of exercises where you need balance. For leg raises, you work your abs by using ab strength to hold your legs up, and as you lower closer to the floor, you need more strength to keep your legs up.
How to do the exercise:
Start in a lying position on your back. Place your palms under your butt with each palm under each butt cheek (left palm under left and right palm under right) facing the floor. Alternatively, you can just place your palms face down straight at your sides. Under the butt gives a little more support.
Lift your legs up to about a 90 degree angle to your upper body, so your heels are facing the ceiling.
Slowly lower your legs towards the floor keeping them as straight as possible without locking out your knees. Do not lower all the way to the floor but hover right above the floor keeping your abs tight and your head on the floor. Notice how you need a lot of ab strength to hold this position.
Raise your legs back to the starting position and repeat. Try to keep your knees together throughout the exercise so you are not lifting and lowering one leg at a time.
As always, here is a great video showing how this works:
Cheers Eights & Weights!
Photo credit: Seventeen.com