For Zein, guiding was like opening a faucet, sending out a steady stream of water to flush the filth that was the corroding miasma. In his most controlled state, the water would come out constant and steady, and the espers would feel the comfortable sensation of a very stable ride.
The first time that steadfastness broke after more than a decade was, probably, during the cave incident, where Zein had to guide Bassena in frenzy, pouring down everything harshly due to the time constraint. But he had no leeway to remember how it felt amid all the pain and the frenzy.
The second time it happened, it was brief and clear. He felt the rush, the way the stream came like a torrent, and had to stop from the sheer shock of it.
The next time he kissed Bassena, however, Zein had prepared for it, and had gained control over the instinct. So he thought, when they actually had sex, he'd be able to control it too.
He was naïve.