Bad Ideas explained: http://www.furaffinity.net/journal/321008/ Includes index.

Better keep it down, fella.

He's nude, bound, and in quite a predicament. His balls are well trapped and upon rests a board or the end of a lever. Hanging from the lever is a large bucket or tub. This adds a little pressure, but only a little. Above the tub is a hopper full of.. a rather heavy something... a heavy something that can flow, though not terribly quickly, into the tub below. At the base of the hopper is a sort of valve. It's very sensitive, a hair trigger setup one might say. And this particular trigger is rigged to the fellow's currently flaccid penis. If he gets sufficiently aroused, the rigging tightens and pulls the trigger. The valve opens. The ..something.. flows, slowly, agonizingly, and relentlessly into the tub below.

Oh, gee is she coming on to him, or putting on something of a show? That could make things... difficult.
No, he better not close his eyes. If he does that for much longer than a blink, well, he better not do that.