פירוש על סנהדרין 48:18

Gemara: Asmachta (literally "reliance") -- term used in the Talmud for an agreement based on speculation. In general, an asmachta involves one party (or both) consenting to surrender to the other a certain sum of money, depending on the performance or outcome of a particular event. In each case, the individual obligating himself enters into the agreement "relying" that the outcome will be favorable and that he will not have to pay. Since a transfer of ownership requires that the object's owner give it willingly, an asmachta agreement is non-binding, because the obligated party never sincerely meant to obligate himself.

it is non-binding -- Rami bar Chama considers gambling an example of asmachta, because each player consents to the terms of the game only because he expects to win. Since the loser does not willingly surrender the wagered amount, the winner is considered to be stealing when he collects, and is thus ineligible as a witness or judge.