Do not lock the joints!

When doing the form, one should not lock his or her joints at all.  Aside from the "external" aspects of it - where locking a joint makes it more susceptible to being hurt or in the case of an actual combat situation making it a good target for "breaking" - there's the flow of energy.

The whole concept of silk reeling is around making the flow of energy (qi) easy in the body.  The energy moves from Dantian to other parts of the body in a spiral way, like a screw going into the wall.  In order to do that, the muscles and joint should let the energy flow easily.  As I mentioned before, some of the silk reeling exercises are actually designed specifically for that, to open certain parts of the body and enhance the flow of energy.

Locked joints are the opposite - the do not let the energy flow freely.  So by locking a joint one is in fact blocking the energy and preventing proper circulation of qi.  This is specially important in the case of knees and elbow, because the form makes you stretch your legs and arms quite a bit.  So if you're not careful, you can easily lock the joint as you're stretching.