The ins and outs...
You: lie on your back on a flat, elevated surface (desk, dining table, etc) and place your hips at the edge of the piece of furniture. Lift your legs so you make a right angle and let him do the rest...
Him: stood up, facing you, he grabs hold of your ankles. He spreads your legs apart and enters you at the same time, then he crosses and recrosses your legs, thrusting continuously.
Why is it so good? This scissor movement gives a range of sensations. When opening your legs, you'll appreciate the depth of his penis inside you, and when he closes them, you'll go crazy as you feel him imprisoned between your legs.
A good time to try it: When you trust your partner enough to give him such a gratifying view of your intimate bits! It's also a good way to catch up if you missed your weekly abs class!
Tip: Don't just leave your hands dangling...if you can reach his bum, caress it to give him an extra thrill!