The water in the sink is going down, just slowly enough that you notice. You have poured something down there before. It helped for about a fortnight and then you were back to watching a basin drain while you brush your teeth.

Most of what people pour in never reaches the problem. A thin liquid follows the fastest path it can find, which is the wall of the pipe and the gap the water is already getting through. The clog is usually below the trap, sitting in standing water, and a runny cleaner drifts past it and dilutes itself on the way. You dosed the pipe. You did not dose the blockage.

Density is what fixes that. A thick, concentrated formula sinks through standing water rather than mixing into it, so it arrives at the top of the clog and stays there long enough to break down what is holding it together, which in a bathroom is hair bound in soap and in a kitchen is congealed grease.

Which means the instruction that feels wrong is the important one. Do not run the tap after. Water is what dilutes it, and flushing it through early is the most common way people get nothing for their money.

Here is the limit. This dissolves organic matter. If what is down there is a bottle cap, a child's toy, a build-up of scale, or a collapsed section of pipe, no chemical is going to touch it and you have a mechanical problem. Repeated dosing on a mechanical blockage is how people end up with a pipe full of caustic and still no drainage, which is a worse day for whoever opens it.

It is also the wrong tool for a fully stopped drain. If nothing is moving at all, the product cannot reach anything past the standing water it is sitting in, and a hair tool or a plunger comes first.

And it does not stop hair going down the plughole tomorrow. This is a periodic job, not a repair.