The proper response to that realization is to unwind the tension, not to keep pushing. Really, he just got lucky, but that's fine, it's an interesting video. I take risks myself when building stuff but I would not trust my life to something that isn't engineered for strength and in spite of all of the work that went into the computations of how to get the projectile up to speed as well as the strength of the arm there is already some proof that he misjudged how strong wood is for this application (one of the arms desintegrated).

Wood is super strong, if used properly, this looks like a setup similar to the neck of a base guitar but without the engineering of the joint to go with it, and a much nastier angle between the direction of pull and the joint.