Yes, laid on bias is changing the angle of the cloth on the boat. Has to do with the orientation of the strands of fiberglass in the cloth.

Some will claim that fiberglass cloth loses all its strength with just a slight bias. Nothing I have seen from any engineering documents on the subject has ever lent any support to that claim. More like a 30% reduction at 45 degrees, maximum strength loss. Something that some here forget here is that cloth is made up of a weave, and when you reduce thread count in one direction by turning the cloth you increase thread count in the other direction. At 45 degrees, you've got 1.4 times the thread count then when you're parallel to the fibers, if the cloth is uniformly woven. That's significant if you only do calculations as opposed to actual direct measurements of strength, and often improperly overlooked.

There are many many boats built with cloth on the bias precisely for the reasons of fitting a curve. For a cloth laid on bias so that it will fit a sharp bend is far stronger then a cloth made of broken fibers from trying to crease the straight laid cloth into too tight a curve. Broken fibers are never strong.

You also must understand and remember that stresses to the hull are not inherently aligned with the cloth or the direction of travel. Just because you are paddling forward does not mean when you hit a rock that all the stresses go fore and aft. They will in fact go many different directions due to many other variables. Hull shape, point of impact, rolling action, etc.