You don't have to be very strong to give a deep massage. This stroke uses the ability to wedge your hand between your partner's back and neck and the massage table or bed. Their own body weight will help push downward.
The muscles that run behind the neck form a sort of diamond pattern. The muscles that run up the neck are thick and respond well to kneading with the thumbs. The muscles that run downard to the spine are thinner and can be kneaded with the heel of the hand.