First learned it in the Marine Corps, with an M-16, and where magazine retention is stressed primarily because of extended operations and little chance of finding a supply depot out in the desert to issue new magazines.
Since then, I've found at least one shooting system that takes the time to explain a variation on the traditional Tactical Reload - which is synonomous with "magazine retention".
The C.A.R. system explains this variation as a Tac-Com (Tactical/Combat), which is simply a hybrid of the traditional definitions of both Combat Reload and Tactical Reload.
Many people here arguing against the Tac Reload have a very valid point - sometimes magazine retention becomes the primary focus to the exclusion of the real goal: topping off your mag without running to slide-lock. In all honesty, it just doesn't make sense to say "I will ALWAYS retain my magazine". Sometimes, you're just going to want to make sure there's a full magazine in the weapon and you're not going to care about where the old one falls.
I hear and agree with the arguments that always worrying about where your spent mags are will end up badly for you one day. But, then again - do ANY of us here advocate ALWAYS doing something the same way, or do we prefer to adapt to the situation? Same here.
Koz posted a video earlier in the thread and stated that the example wasn't a Tac Reload. Maybe not, but it certainly does fit the description of C.A.R.'s Tac-Com...dropping a used mag in favor of a new one but not worrying about retention.