In the karting context, flexing is what the frame does when cornering, since there is no suspension on a kart, and there is not differencial on the rear axle. As a result the inner tire in a curve would scrub. The flexing allows the inner tire to slightly lift off the ground, reducing tire scrub. Stiffness, i.e. the flexing ability of the kart, greatly influences the handling and can be changed through various means.