Underwater videos have shown carp shaking their head in order to swing the lead to eject the hook.
The Boa Rig is designed to combat this. As the carp shakes its head the Boa Rig stretches and the lead stays on the lake bed. The hook then pulls deeper as the Boa Rig contracts ensuring a strong hook hold. The carp realises it is nailed and cannot eject the hook and the result is a screaming run!!!