Muhael’s Spell Siphon is a dedicated counterspell. It can only be cast from a readied action to interrupt spellcasting. Rather than directly undoing the magic as it takes form, it draws the spell energy away from the spell, and toward yourself, allowing you to absorb and repurpose it, re-energizing a spent spell slot up to 4th level, or the level of the countered spell, whichever is lower. If Muhael’s Spell Siphon is heightened, it can restore higher level spell slots, up to the heightened level -1, although the restriction based on the countered spell still applies.