I just recently started playing Assassin's Creed: Brotherhood on the PC. When I started Sequence 2, there is a mission where I have to tackle a guy who stole my money. I keep chasing him, but I can't find a reliable way to tackle him.
The "Shift" key contextually changes from "Shove" to "Tackle" but I can never manage to consistently execute the tackle. In general, I leave it up to chance and somehow I get it to trigger. I also can't figure out how to shove people either. Usually I just end up falling into them and tripping.
I had this same problem in Assassin's Creed 2, but I managed to get through the whole game without really needing to perform either of these moves.