Definitions by Rupert Benedict

helmet kiss 

The action of a pillion motorcycle passenger knocking helmets with the motorcycle rider.

For example, this may be caused when the motorcycle is under unexpected braking or slowing down when the pillion may not be aware of this and is unprepared.

Not quite "docking" although that could be an alternative definition of a helmet kiss!!

helmet kiss by Rupert Benedict June 20, 2006