The latest hotfix list contained a cryptic message about a bug fix involving high Greater Rifts.
Fixed an issue where players could sometimes enter a high level Greater Rift with monsters that have very low hit points.[source]http://us.battle.net/d3/en/forum/topic/18300625289#4[/source]
Lots of players wondered what that meant, and thanks to a useful video, now everyone can know. The trick was to clear a low level Grift as quickly as possible, and then enter a normal Nephalem Rift immediately afterwards, sort of sneaking back in before the game closed the Grift. The player could then very quickly clear the entire Rift up to the Guardian, portal out, start a Grift on their highest possible level, and find themselves on the same already-cleared map, with the same low hit point Guardian waiting to die... and with hit points appropriate to the low level Rift.
This exploit was how we kept seeing top PTR Leaderboard times of like 2:31 for GR65.
Click through for the video showing off the entire process, and thanks to Flying Dragon for creating it.
Record GRift Exploit Demonstrated
Continue reading the Original Blog Post.