Though you can cast spells, you do not regenerate spell slots when you rest. Rather, you absorb the spells used against you.

When you get hit with a spell that uses a spell slot you also have, rather then be effected by the spell, you instead gain the spell slot for your own use. However, you can't absorb more spells then you currently have slots for. For example, if you only have 2 1st level spell slots, you can't absorb a 3rd 1st level spell, or a spell cast with a 2nd level or higher spell slot.

Additionally, you can't absorb a spell if it targets more then just yourself, for example, fire ball, but if it effects only you, even if it can effect others, like bless, you can absorb it.

If you are unable to absorb the spell, you are effected as if you didn't have this feat