When you activate it, several areas of grass will shake. When you go to them, a Pokémon will appear. Defeat it and it will commence and continue a chain and more areas will shake. The more a patch of grass shakes, the rarer the Pokémon. If you continue the chain, there is a hidden bonus of the possibility of a shiny patch with the probability topping out at a chain of around 40.
There are a few changes to the PokéRadar, however. First off, there is not a separate encounter table for the Pokémon in the area. As such, you will only encounter the Pokémon you have already seen. Secondly, there are occasionally decoy patches. These patches will shake, but when you approach them, it will tell you that there is no Pokémon there and your chain will be broken
Hope I helped!