you have a muscle in your groin called the cremaster. Its sole purpose is to lift the testicles. The cremasteric reflex involves sensory fibers in the inner thigh, which you can stimulate by lightly stroking, sending information to the spine at vertebrae levels L1 and L2. They synapse with motor fibers that bypass the brain and activate the cremaster. In other words, stroke your inner thigh and you pull your balls up. It can be used (but is often not, for obvious reasons) to diagnose problems with spinal or peripheral nerves at levels L1 and L2.