You poured the bottle in. You waited the twenty minutes on the label, you ran the hot tap, and the water went down a bit faster for about a week. Then it was slow again, and the second bottle did less than the first.

Here is what is actually in there. A shower or basin clog is mostly hair, bound together with soap residue, and it is not sitting flat at the bottom of the pipe. It is wrapped around the crossbar under the plughole or caught in the bend, hanging like a wick. A liquid poured from above runs past it, follows the wall of the pipe, and spends most of its contact time on whatever is downstream of the actual blockage.

A barbed strip works on the one property hair has that chemicals struggle with: it is fibrous, so it can be hooked. The barbs point back toward the handle, which means the strip goes down past the mass easily and grips on the way up. The pulling is the whole mechanism.

So the job is short and unpleasant rather than long and chemical. Feed it down until it stops, twist a quarter turn, pull steadily. Have a bag ready, because what comes up is genuinely worse than you are imagining and you do not want to be looking for a bin while holding it.

The limit is honest and it is about geometry. These reach the trap and not much further. If your slow drain is a kitchen sink clogged with congealed grease down the line, or a blockage past the bend, there is nothing here that will touch it. This is for hair, near the top, and it is very good at exactly that.