The effect includes Chests already spawned. Entering a room that previously contained other types of Chests while holding The Left Hand will turn them into Red Chests permanently.
Chests that have already spawned are not converted into Red Chests.
It is advised to drop The Left Hand (by holding either CTRL key for few seconds, holding R2 or RT for 3 seconds on PS4 and Vita and Xbox One, respectively, or holding ZR on the New Nintendo 3DS / Nintendo Switch for a few seconds) before entering Chest, as non-Red Chests always contain item pedestals on this floor, especially the 4 located in the starting room.
Isaac may prefer Red Chests for their possible Soul Hearts or Red Chest items, at the expense of guaranteed items.
Locked Chests converted in Chest's starting room will contain Devil Deals, potentially allowing for more powerful items, but at the cost of Hearts.
Red Chests found past the starting rooms of either Chest or Dark Room will contain regular Red Chest drops. Dropping the trinket before leaving the starting room is advised.
The Left Hand is a reference to occult beliefs in which the "The left hand path" is associated to black magic.
The Left Hand is the only item in the game with 2 separate conditions for unlocking it, being unlocked by killing Ultra Pride, or by defeating ??? in Chest as Judas
For the trinket with a similar name, see Monkey Paw.
Red Chests.
The effect includes Chests already spawned. Entering a room that previously contained other types of Chests while holding The Left Hand will turn them into Red Chests permanently.
Chests that have already spawned are not converted into Red Chests.
Chest, as non-Red Chests always contain item pedestals on this floor, especially the 4 located in the starting room.
Soul Hearts or Red Chest items, at the expense of guaranteed items.
Locked Chests converted in 
Bomb Chests are affected by The Left Hand. 
Old Chests and 
Mega Chests are not affected.
Gilded Key: Overridden by The Left Hand.
Sharp Key: Enemies killed by keys will only give the contents of red chests allowing items to be obtained through Sharp Key more consistently.
Ultra Pride, or by defeating 
??? in 
Judas
Judas Unlocks 
??? (Boss) Unlocks 
The Binding of Isaac: Rebirth
