Must admit that is an aspect that confuses me too. If we think of willow in it's natural state it is very soft, incredibly so for wood. Pressing will have a hardening effect, compressing the soft fiberous wood into a smaller volume, then knocking in will do a similar thing, compressing the face further. I don't understand how a bat can then become softer through use. All playing in is a form of further knocking in, surely? Both processes involve hitting the bat with a hard object and potentially compressing fibres more so.
Similarly, I don't follow the logic of batmakers who are known to press the bat harder than average who say that the bat will take longer to play well. Surely if a bat is harder to start with, it isn't going to get better the harder it gets (through knocking in, or use)?
There was talk a while ago that soft pressed bats have ultimate initial performance, I get that. I don't see how it works the other way around though.