First, your question.
Crossbows at that time would have been organic materials, either wood, or horn and sinew. Leaving them strung continuously is detrimental. Therefore they were equipped with a second "nock" just past the the nock that the working string sat in. This allows the archer to put a second, longer string called the "bastard string" on the bow. Drawing the bastard string takes the tension off of the working string. The loops of the working string are big enough to slide out of the nocks and down the limb of the prod while the tension is off. The length of the bastard is such that it can be removed while the prod is relaxed.
Second, my rant about Crecy.
I never liked the whole "wet bowstring" explanation. For starts, if you wet a string made of linen or hemp, it does not get longer. In fact is gets shorter. I learned this the first time I thought I would bind in a prod with damp linen so that it would shrink down tight. It didn't. It got looser as it dried.
Now, on the other hand, dampness is very detrimental to the performance of sinew backed bows. If we assume that most of the crossbows in Genoese hands had composite horn and sinew prods, then a good rain might have given them trouble. This is not really a satisfying explanation, though. The composite prods that have come down to us all have or have had a covering of painted birch bark to prevent water from soaking in.
Then there is the question of why this story is told only of Crecy and no other battle. Surely there were many medieval battles fought in the rain. Why would Crecy be the only one where this is a problem? If makes the whole story sort of suspect, and I am inclined to distrust it.