Have the customer get on all fours to expose their vulnerable testicles and perineum, with the therapist using their fingertips, nails, and the base of their thumbs for treatment. Then, using plenty of oil, perform a slow hand job on the glans, mainly using reverse motions to take advantage of the oil's slipperiness, bringing the customer to the edge without letting them go over, squeezing gently but firmly. Finally, finish in whatever position the customer prefers, with the therapist releasing the built-up semen with their fingers.
