The basic idea of the bayonet is that you use it in a final charge on the enemy's position, having worn them down with rifle fire, mortar fire and other means. So you are going at the enemy at the run, and the momentum provides additional force. By that time, in fact, the bayonet was very rarely used for its official purpose, having been superseded by grenades and the like, plus all kinds of unofficial nasties, such as homemade clubs studded with 6-inch nails.
When I was with the Army Cadet Force we used to teach 'section attacks', i.e. an attack by an 8-man unit on a position usually held by one or two. The final bit involved running at the enemy with bloodcurdling yells (the cadets particularly enjoyed that bit!) and finishing off the enemy by unspecified means.
Ann