Ok, suppose you are out of status but a family member petitions you and your priority date arrives. The first thing you'll need to do is get your biometrics and fill out the forms. Right now, the two offices are separate so you can go get finger printed, etc. without having to worry about getting detained.

With the new rule however, you'll have to go to CIS to get finger printed while you are still out of status and if they want to, they can check you and arrest you on the spot before you even get the process started. Is it likely? maybe not, but your chances have increased.

And that's just for a simple overstay. Suppose you have no status or had a removal/deportation order and are trying to readjust your status...maybe you were 245i or otherwise have the ability to readjust. Again, you go in to get finger printed and they do a background check and see that you have a deportation order and boom, detained.

The buffer is gone.