you can expend any number of hit points and roll them if any of them roll a 1 or the highest number you can roll you gain a 1st level spell slot per hit die that rolled those numbers.

you can take damage equal to three times the hit dice expended to roll your spellcasting mod hit die without expending them.

at 5th level you can roll 1 hit die without expending at or 2 at 11th level and 3 at 17th level.