Holding smooth skin weights
When you are changing (editing or painting) the weights of smooth skin objects, changing the weights of one object can affect the weights of other objects. This is because Maya must consider the weights of all skin objects being influenced by a particular influence object as being relative to one another. Maya does this by requiring that all the weights add up to one. When you change certain weights, Maya can automatically change various other weights so that the total of all the weights continues to be one. This allows Maya to know the relative influences of the weights.
The process of scaling some numbers so that they all add up to one is called “normalization.” By constantly normalizing weight values, Maya keeps track of their relative influences.
When changing (editing or painting) weights of smooth skin objects, it’s sometimes desirable to specify that the weights of certain objects won’t change. If you’ve perfected the weighting of a particular object, you might want to make sure that its weights are not going to undergo any normalization changes. Maya lets you “hold” the weights of particular smooth skin objects so that their values don’t change when you are editing other smooth skin objects.
Note that if you hold the weights of many objects at the same time, Maya might not be able to normalize the rest of the weights properly, and you could get an error message. In general, you should not hold the weights of many objects at the same time. Typically, you would want to hold the weights of only one or two objects. However, if you don’t want to stop holding the weights of any of the objects, you can turn off weight normalization directly from Maya’s interface (see Controlling smooth skin weight normalization).