Not sure when they added it in-game, think it's been there a season or so at least?
(BTW, this might amuse you -- i had to look up the term "dead-rubber" just now)
Offhand, "garbage time" is triggered when there's a dead-ball (clock stoppage), & you have a 20+ point lead -- from about the last 2(?) minutes of the 3rd quarter on. So basically it's when the game is 3/4 over and it's a blowout. But yeah, the coach pulls all your starters & puts in your reserves to protect your starters from injury & whatnot.
& If the game gets close again (think if the lead drops to 10 or so) then your starters will come back in on the next dead ball.
(Someone else can give you the *exact* numbers on all the above, I just gave a close approximation.)