When in doggy style position, twist your wrist inward to the "thumbs down" position. Place your thumb over her bung-hole and move it back and forth in a circular motion, much like the blades of a helicopter.

A jack off buddy (from 'cum' /'come', meaning 'semen', and 'companion'); a friend that you get together with for sessions of solo or mutual masturbation
Variant of: cumpanion (from 'companion' & 'cum'); jack off buddy & wank bud & JO buddy
Similar to: fuck buddy