Liquid Mask - How's it Work?

Seen it mentioned, but have never seen the product.

Is it used to mask hard to tape contours, etc? Peel it off when done?

It can be used to mask hard to tape contours. I've used it for canopies. I created a "frame" with thin masking tape, and just "painted" the liquid mask in the middle. Peeled most of it off when I lifted the tape.

What brand? I've been using Micro Mask from Micromark - kind of thick, bluish stuff. Don't recall an ammonia smell - on the other hand, I don't use it to mask (say) camoflage. I've started taking to Silly Putty for some of that. <g>

Liquid mask is used by the ton by talentless watercolour "artists".
Every watercolour maker (Maimeri, Schmincke, Talens, Daler-Rowney,
Lefranc&amp;Bourgeois, Winsor&amp;Newton, DaVinci, Holbein, Sennelier, Old
Holland, Grumbacher...) has his own recipe, but basically it's always the same thing: a suspension of latex in a water-based solvent. The green one could be L&amp;B or W&amp;N's; Schmincke, Talens &amp; D-R used to be ivory white. They all have slightly different properties, so you can try and pick the one that suits your needs.
And of course you can find'em in just about any art supplies shop...

I have the remains of a plastic-type liquid masking fluid. It was given to me in a blank container, so I have no idea what it is. Works wonderfully, vile milky emerald green when wet, dries to a thin bottle green as the water evaporates and it polymerises. Any idea what it is and how I can get some more here in the UK?

I've run into another problem with Maskol. As you say, it is latex, which means it stretches when you peel it off. In my case, this caused the paint to chip off the masked area, and this chipping continued just beyond the masked area in some places, resulting in a rather unpleasantly jagged line.
Do other masking fluids also have this problem?