The idea is simple: instead of gear up and push higher keys, it focus on gear down. For a given keystone level, the less item level you have, the more reward you will get.
This eliminated the barrier between a vast majority of undergeared players and high level content. Since every keystone can have a least iLvl record, it also make every keystone valuable as well.
This mode can be named “Echo of the beginning of seasons”, actived at middle of the season, to remember the first day when mythic season came out, the undergeared heros pushing themselves to challenge higher keys.
Most importantly, this changed the default direction of game content: higher content usually requires higher item levels.