Here is my $0.02, basically when that barrier in their mind snaps and the wolf takes over the man, a timer essentially starts ticking. The wolf priests have the ability to command them as though they were a wolf lord and denounce the wolf to allow the man to take control. The first time they go wulfen, it is horrifying to them and they let the wolf have their body. If through sheer force of will (Ragnar's case since he almost wounded his charge) or through commands (being talked down by the wolf priests) the man is able to once again take control of the wolf, they pull through. Every single time the warrior succumbs to the wolf in the future, the wolf gets stronger. Wolf priests teach the man litanies to help control the wolf and to pacify it so that they can try and fight the wolf for control in battle. Basically it comes down to the marine. Some are strong enough to never fall prey to the wolf. It simply becomes a choice for the space wolf to "unsheath" the wulfen inside him once he has come to peace with it. This is, of course, my interpretation from reading the Space Wolf Omnibus for the fifth time ;p reading through the initiation part is the most enlightening