The art of jumping into someone's back when they weren't looking. However, soon after the point of take off (approx 3-4 feet away from the target), the 'riser' was required to turn 180 degrees, sticking out his backside as he did, with the intention of planting it firmly between the shoulder blades of the recipient.

Correctly executed, the initial stages of the riser bear a remarkable similarilty to the approach of the High Jump, with its curved run up and hilariously elongated stride.

Noone got hurt until one day someone inadvertently bent down to pick something up, and Simon Gotch broke his wrist. We were all more careful after that. Well, a bit.