I'd agree with Tessa here, for most of us it doesn't really work to try to strike yourself hard, the way you'd slap another person you were really angry at. Our reflexes militate against it, plus the strangeness of the situation. And another thing, when you're lying bound on a bed, your hands and feet pulled out to the corners, and taking one cruel whip lash after another from an outwardly cold Master, you'll want to go further. Meeting your Dominant's gaze and seeing him do this to you, his painslut, will push you forward because you see that it actually means something to you both. That's the instant bond that's difficult to fully realize if you're doing it all on your own, or even to report about how it felt later.