There is a circular slot on the bottom of the front face, in the relaxed position there is a stop pin that stops the pulley arm from unwinding fully.
when you use a wrench to turn the arm against spring pressure a hole is exposed on the fixed body that attaches to the engine, it is in that hole that the 5mm pin is placed. This gives enough clearance for the belt to be routed around the pulleys, if the pulley arm was in the relaxed position there would not be enough clearance to put the belt on. Watch your fingers when using the pin, if it comes out you may trap your fingers and damage them severely.
Using the original belt with a belt wrap increases the load on all of the bearings of the pulleys, Ideally the idler tension arm should be in the same position as the OEM position to match the lives of the bearings. Some of these bearings are in expensive items, water pump, AC, SC pulley etc.
Extra tension on the belt will cost you in the end, sooner than the OEM setup.