How do you actually enforce an Assignment Of Rents in a mortgage?

[Wayne Brooks]

This may sound simplistic, but I'm looking at trying to buy NPN's on a 4 plex or two. I would like to collect the rents while the mortgage is working it's way through the foreclosure process, to off set costs. If the mortgage has an Assignment Of Rents, I obviously would have to give some type of notice to the tenants and owner, but how could I enforce this upon the tennants, assuming I have an uncooperative owner?