The ins and outs...
You: lying on your back, legs stretched out and arms up towards the headboard. You seem to be offering yourself to him passively...but wait...
Him: he positions himself above you to make love missionary style, but for a break from routine, you tighten all your muscles while he's doing his thing! Squeeze your thighs and stretch your arms above your head, as if you're trying to push the wall.
Why is it so good? Stretching all your muscles and squeezing your thighs very tightly multiplies the effects of penetration and naturally stimulates your clitoris. Surprised by your physical resistance, he'll redouble his efforts!
A good time to try it: When you're knackered after a day at work and you've forgotten to do your daily stretching exercises. Relaxation, exerciset and lots of pleasure...what more could you want?
Tip: For variation, stretch your legs out as far as possible towards the end of the bed, keeping them squeezed.