This must be your first AC game. Scaffold take downs were particularly handy in AC1 since Acre had so damn many of them and bunched up guards allowed you to take out 3-4 at the same time. --- Alice returns in 2011, I'll be ready... http://www.ea.com/alice Inception ending spoiler: http://twitpic.com/26k9ub
As the above noted, throwing folks into scaffolding to kill them (when it collapses on them) has been in the game since AC1. AC2 gave you lots of places where it was extremely useful and introduced some enemies that will throw YOU into the scaffolding if they get the chance.
So, like a lot of things you're asked to do at the beginning of Brotherhood, they're really just checking to try to ensure you know about all of the things you have available to you. Doing it is simple. 1) make sure you are unarmed (Down Dpad for PS3); 2) kick him (upsets his guard); 3) grab him (attack with free hand using counter-timing, or hit it right when he's attacking you); 4) drag him close to the scaffolding, or at least where he's in tossing distance (12 feet, maybe); and 5) toss him into the scaffolding by pointing the LS in the direction of the scaffolding and hitting the free hand. Done.