Complex sentence: Weights may be hung from the clamps to increase the pressure on the nipples, and to discourage the clamped person from moving suddenly (for example, jumping when spanked or caned), since this would make the weights swing and increase the pain.
Split into simpler sentences: Weights may be hung from the clamps to increase the force on the nipples. This also discourages the clamped person from moving suddenly (for example, jumping when spanked or caned), since this would make the weights swing and increase the pain.