Heine is already wary, sensing that Jade's discomfort at this entire procedure is growing. As a result he's on guard enough to avoid much of the blow. Still, a part of it lands, and he reels before pushing onwards.

Nothing ventured, nothing gained. And it's not the worst he's ever suffered.

In Jade's memories Professor Eichel elaborates on the spell he'd placed. It's simple. Whatever you want the most... it'll feel like that—and it's hard to listen when Heine can feel everything Jade had, then.