The last sentence means that the bowmen A contacted in the flank must not slide forward to allow front corner to front corner contact.
It stay put because the side contact showed is a legal contact and thus this is not necessary.
At the end of the enemy movement phase, If A will be contacted on the front too, X will stay in place and combat will occur. If bowmen A front will not be contacted, A wil turn to contact X front to front as normally happens when just side is contacted.
