If she knows what she's doing, it feels great. If not, it'll hurt like the dickens.

The prostate is the male gland that produces seminal fluid.  It can be stimulated two ways.  Externally by applying pressure on the perineum and internally by inserting a toy or finger anally.  If a man is lying on his back, it would mean inserting a finger a few inches in, and stroking the roof of his rectum.  The prostate feels like a bump.  It's distinct.  And LOADED with nerve endings.  The sensation can be quite intense and it can also bring on orgasm very quickly.

The ad is most likely referring to internal prostate massage.

Dude, that means that she's going to stick her finger up your a$$, not the other way around. Do you know anatomy? Women don't have prostate glands. Like my doctor told me" put your elbows on the table and take a deep breath".