The spellcards that you know you can do, but just barely. You're a pixel away from clipdeath the entire time, your heart is racing, and your finger hovers over the bomb button.
For me, Kogasa's midboss spellcard in Stage 2 (Hard/Lunatic). The bullets are so tightly packed, and since I never use Reimu, I'm always afraid of sudden clipping (which then leads to clipping, of course :V). Sakuya's Eternal Meek on Lunatic also does this, as well as (surprise!) Kogasa's first EX midboss spellcard.