Here's how you play:
Have partner 'A' lay on their stomach on the ground
(in our case, this is Bob)
Partner 'B' stands on Partner 'A's back
(so Claire stands on Bob's Back)
Partner 'B' then decides on a word that Partner 'A' must yell out the same time as Partner 'A'. But Partner 'A' has a different word they'll say.
(Claire will say something like, 'You say 'Oy' and I'll say 'Unicorn')
Partner 'B' then yells out, 'READY?'
Now here's the tricky part... Partner 'B' will then jump in the air and then land on Partner 'A's back in a sitting 'straddle' position. When 'impact' is made, that is when both Partners need to yell out their word.
The beauty of this game is that the word combinations are endless. So realistically, this game could never end. But some how I have this feeling that the saying 'Its all fun and games until someone gets hurt' most definitely applies... so please use caution when playing in groups (especially groups of people you don't know).
And that, my friends, is how you play '91'.