In the tube. Nigh-invulnerable skin. G-gnomes touch him lightly, he cannot feel it, not on his body, only in his mind. Strangers come. They hit and kick. He cannot feel it. And then the bolts of electricity. He feels those. They sparkle through his body. They are dangerous. He feels alive.
He is out. He is alive. He begins to feel softer sensations, the breeze in his hair, fingers brushing through it. But still in sparring sessions the bright jolts of electricity are like nothing else. The intensity of them teaches him the fine line between pain and pleasure.
Eventually he goes to Aqualad outside of training and asks him to do that more.
When Kaldur - Aqualad no longer - chokes his hand around Superboy's throat and shocks him, it still feels exactly as it always has. Superboy knows that's deliberate.