Purpose of relieving moulding

[Glen Butler]

What is the purpose of relieving knives?

[Jim Becker]

Relieving the back makes it easier to work with the molding since in architectural settings, it's very rare that the surface the molding is being applied to is perfectly flat and true. It also helps with movement by providing an airspace so moisture isn't trapped behind the molding. Flooring is also handled this way.

[Glen Butler]

So it sounds like it not only helps fitting pieces but also prevent warping? If moisture can get trapped as you say, then that could cup the flooring.