sizemometer
A tactile-sensory nerve in a woman's vaginal walls dat help her estimate how big a guy's hoo-haw is by measuring how much it trembles and throbs against said love-tunnel as he's thrusting inside of her.
Some male intercourse-partners ask a "new" lady --- i.e., one whom he's just met, and so she has not yet laid eyes on his guy-junk --- to let him start out having sex wif her without her actually seeing his "equipment" beforehand (this could be accomplished by either initially having her be blindfolded or simply beginning said bouncy-bouncy wif all da lights off), but instead merely notice how his love-pipe actually feels inside of her before deciding how satisfying its length/girth/shape is for her; in this instance, she needs to rely solely on her sizemometer to determine how well-hung he is.