complete stranger

The act of masturbating with both numb hand and penis, giving the effect of watching someone else masturbate.
The logical graduation from a reverse stranger

When you and your partner are holding hands and then proceed to jerk your dick off but without letting go of eachother's hand. For example if you're right handed and your left hand is holding on to your partner's right hand, open the palms up with your fingers still interlocked with each other and then place the hands around your penis and proceed to jerk yourself off.