This is a very erotic and exciting position which allows the man to penetrate his partner slowly and deeply. He sits down, with his erection ready to receive his partner, who then sits on him, perhaps teasing him by playing with his penis, using it to stimulate her clitoris before she lets him into her body. As she plays with him like this, he may get more and more aroused, until the sexual tension is so great that he simply wishes to possess her completely! The longer the build-up, and the more erotic the foreplay, the more powerful and exciting the man's orgasm will be when he finally lets go and explodes into her!
The male partner relaxes with his back well supported so that he can form a comfortable place for his partner to sit between his knees, which are raised slightly off the bed. His feet must have some purchase, so that they don't slide down the bed when his partner sits on him - the whole idea is that his body forms a couch for her to relax into. He can penetrate her easily, and the more she raises her legs, the deeper he will be able to enter her. Note the position of her thighs and his head, and the couple's hands, both of whom have clear access to her clitoris, making it possible that she will reach orgasm during intercourse. This is a great position for men who ejaculate quickly, as the erotic connection will keep him erect even if you rest still very so often. That will help him to control his arousal and slow down his progress towards orgasm.
To enjoy this position you need to be on a hard surface - a bed will rock too much and you may not be able to stay upright. The man sits as shown, holding his legs behind the knees so that he forms a stable base for his partner to sit on. She makes herself comfortable, resting some of her weight on him and some on her legs, taking his penis inside her vagina as she lowers herself onto him. In this position, either he or she can move, but it's a good idea to decide who is going to rock or thrust before you start, since if you both try and move out of sync, things can get a bit confusing. Obviously the man has easy access to his partner's breasts, which he can kiss and caress as the couple make love.
The X factor
You start in the chair position shown above, this time on a soft surface like a bed, then have the man hold the woman's wrists as she lowers herself down onto the bed between his legs. Look carefully at the picture so you get the angle of your legs correct, otherwise the position won't work! The idea is that the man can pull his partner towards him in a rocking motion, though to do this he must be fairly strong and she must trust his ability to hold her. It's a great exercise in trust, and fun to play with, though you may find it is tiring and you are better returning to the chair position above, perhaps using a variation where you hold each other around the back to support each other in a sitting position (see another form of this sitting position below).