Yes, you can use your forearm in boxing.
The reference mentions a boxing technique where a fighter weaves under a punch (specifically a right hand) and then uses their forearm in a particular way. The excerpt states, "He's weaving under the right hand. And then he's putting that form right underneath me there okay." This implies the forearm is being used defensively or as part of a close-quarters maneuver. While the context is limited in this single reference, it demonstrates that the forearm can be utilized within the rules and techniques of boxing. The information does not explain the intricacies of the technique or the legality of all forearm uses in boxing. Therefore it is important to follow all the rules while boxing.
