The idea is to insert the blade until it doesn't go any further, then whilst trying to open the gap a bit further by bending the blade outwards, slide it down the side. You shouldn't need to put any pressure on the tabs, which you would do by forcing the blade further in.
The tabs actually extend over the gap, so when you stick the putty knife in all the way, you are actually hitting the tabs. As you pry, you apply pressure against the tabs and can cut them or break them if the blade is sharp enough.