Recoil, by definition is "to spring or fly back, as in consequence of force of impact or the force of the discharge".
In Beyblade, Recoil is referred to as the often undesirable result of a collision between two parts, in which the Beyblade is knocked in the opposite direction after colliding with its opponent, resulting in decreased Stamina and a possible self-KO. Prominent contact points that make contact with another Beyblade is usually the cause of Recoil.
Recoil is not to be confused with Smash Attack however, as Smash Attack is used to try to Knock-Out your opponents Beyblade, where as recoil is a negative result of such a collision. Although Recoil and Smash Attack capabilities are often linked, the most effective Smash Attack Wheels are ones which do not suffer from recoil. In Beyblade Burst however, certain high recoil parts are better at bursting opponents.