YO!? I'm using the water [light].

In older houses like mine water pressure is not sufficient to do pressure critical things all at once. The chief example is when one person is showering: any other users will upset the balance in the hot and cold water. Scalding or chilling the person in the shower.

Now you could, I suppose, hook up some pressure increaser or any of the lovely antiscald devices, but that is asking for trouble, the pipes might not take it and there you are with cartoon-style spraying leaks all over the house and some 'splainin' to do to the spouse and the after-hours plumbers.

Instead you use the YO! light. A small pressure switch (like the oil pressure sender from my 1969 VW Bug but adjustable) is introduced into the system and when the pressure drops (as in when an appliance is using it or someone is in the shower) it sends a simple radio signal.

The receivers are sealed plastic rings with red LED's that go on when the pressure drops. Occasionally a yellow light comes on to say it needs to be recharged in the induction charger. But this is infrequent since the LED is a low power draw and there is no button or flashing etc.

How to use: You can either wear the plastic ring around your wrist (overkill but if you regularly scald your loved ones it is a small price to pay) or just drop the rings over the kitchen spout or any tap handles that might tempt one. Just put them wherever it would best be seen before turning the handle.

For instance: when at the kitchen tap you reach for the handle, see the red light and you think...

That's the back garden's timed sprinklers, no big deal, I can turn the handle.
That's the dishwasher, it is stupid and will run poorly with less water, I'll wait.
That's the Fembot, I'm not sure she's waterproof...
That's the brother-in-law, time to hit the toilet lever NOW.

The advantages as I see them are:
Minimal invasion to the plumbing system
No risk of overpressure for an old system
User can still override (for whatever reason)