The posterior fornix is that area of a woman's vagina below the cervix, in the back, and down. Think of it as the floor of the back room. This is the 'home' of the 'vaginal orgasm' as opposed to the clitoral and G-Spot orgasms. Female ejaculation can occur when this area is stimulated.

Properly stimulating this area begins with making sure she is not only aroused but is also post-orgasmic. This way, her vagina is fully lubricated and has grown to tis fullest extent making access more comfortable for her.

In the male superior position, lean forward over her put your legs further back and think downward. Gently, at first, caress the posterior fornix with the head of your penis gradually increasing speed and pressure as her orgasm approaches. Modify your "approach" for other positions.

This is especially useful when going for multiple orgasms. You can move from the clitoris to the G-Spot to the posterior fornix, in rotation, moving to a new area after each orgasm for as long as you can maintain. This will prevent one area from becoming hyper-sensitive. Each woman varies in how she is made/wired and in what she likes so be prepared to modify your technique as required/requested.

Have fun practicing!