Deaf-mute sex

A young deaf-mute couple gets married. At first, they have sex with the lights on, in order to sign to each other.
One day, the woman asks, "Can we try to make love with the lights off?"
The man says, "OK, but how will you know when I want to make love?"
The woman says, "Well, when you’re in the mood, just shake my left breast once, and I’ll know. If you don’t want to, shake my right breast once."
The man says, "All right. And if you want to make love to me, shake my penis once, if you do not want to make love to me, shake my penis about 50 times."