When passing "pset.xp" to a player in the following manner, leftclicking on an enchanting table with a glass bottle:

otherdrops:
ENCHANTMENT_TABLE:
- tool: GLASS_BOTTLE
drop: 384
consumetool: 1
pset.xp: -5

If you're at level 21 and a half and repeatedly click, your experience bar will reduce until you hit level 21.0, then stay there while still coughing up XP bottles, you won't go down to 20.