Players can bet their acquired parts on one of two competing players in an attempt to add a magic bonus to their part(s). If the part doesn't have any magic bonus prior to the bet, it'll be assigned a randomly chosen one. The amount that the magic bonus is raised depends on the player's chosen tier to bet on.
Pick the car that gives the smallest bonus, that car generally wins, unless its a 2% on 2%, if so, may god help you. Also, in the case of an unstable car with high damage vs classic with low damage and forklift/repulse, choose the classic.