When receiving a handjob, the act of stroking the penis slowly and gently then stroking it quickly with great speed causing a vibrating effect, that ends with the head extending over the foreskin and finally ejaculating all over the hand. Very similar to letting a dog outside then scratching its belly quickly and it's mouth slobbering all over your hand. This is not limited to a having partner spot you, as you can in fact do this by yourself in the act of masturbation. However, this sexual act cannot be performed on a circumsized penis.

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