using the effect can reset player data to their last death, this includes exp, inventory and e chest.

how to dupe using it:

(it is suggested to use it in a potion form while doing this)

firstly, put the items you want to dupe in your inventory or enderchest

second, kill yourself in game(do not use the effect to do it)

third, respawn place the items in a chest or a container

fourth, throw the potion on yourself

result, you will respawn with the items you had from the previous death and the other titems will remain in the container you placed them in

deleting player data:

from the dupe example you can see this can overwrite playerdata so by using it you can effectively set someone back awhile and reset their exp since their last death