Metroidvania where each upgrade modifies or disables a particular form of game-mechanical bounds-checking.
For example, one upgrade might disable underflow checking for the player character's heath meter; most damage sources only inflict one "unit" of damage (assume a very coarse-grained health system, like Legend of Zelda hearts), so this isn't terribly useful in the general case, but against any damage source which inflicts 2+ health units in one go this can be exploited to make your health wrap back around to full if the final value would be negative. Apart from its situational applicability in fights against bosses with highly damaging attacks, this also serves as the game's equivalent of a hellrun mitigator; damaging environments knock off two units of health per tick, so as long as you ensure that your current health value is always an odd number it will just keep wrapping around indefinitely.
Another upgrade might uncap the player character's falling velocity. Mostly this just lets you get places quicker, but in locations where it's possible to fall long distances it can get you going fast enough to clip into the floor. The game's progression logic explicitly incorporates these floor clips, and several areas are gated by long vertical shafts with one-tile-thick floors at the bottom where the only way to pass is to use your uncapped fall speed to clip through.
Late in the game you pick up an upgrade which disables flag checking for scripted events, making all events available regardless of whether you've satisfied their prerequisites or not. This is mainly useful for completionists, since it allows map areas gated behind "permanently missable" flags to be freely explored during the 100% cleanup phase, but it also grants access to several events whose prerequisites are impossible to satisfy during normal play, including story events which require multiple mutually exclusive outcomes to have happened simultaneously. The narrative implications of this are not addressed.
The last upgrade, of course, disables the map position check that automatically kills the player character if they're out of bounds. This is necessary to reach the final boss.
