If the chap at the bottom has a greater moment (mass x distance from rope), then his weight will rule.
So, 1. the bike does not fall off ever, falling involves "raising" the bottom, which has a greater moment.
2. when they spin around, centripetal force exeeds gravity, in the direction of the "bottom" chap. Again, due to him, they never fall off.
In simpler words, the bottom (as always :) is really the one who is controlling the show, even if he seem to be just a mere passenger.
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