-> Decent introduction here
-> Blizzard also briefly described it in the recent AMA

TL;DR: Some of the new legendaries and trinkets introduced in 7.2.5 use a deck based RNG system which guarantees exactly X procs in Y casts. This greatly reduces the volatility of the items compared to using pure chance and makes them especially favorable for healers and tanks who prefer low-variance items. There is no in-game indication of which RNG system an item uses and we now have multiple "psudo-random" proc systems on items (rppm, Deck of Cards).

What do you think of the new system? We'll probably see a bit of this one.