I think the important thing is to ‘understand how it works’, or rather that there is a little trick to it.
Story one. If you want to inflate a balloon, you need to compress the air working against the rubber. So you would be better with a dedicated pump rather than trying to use lung power...
Story two. If you want to inflate a plastic bag, you aren’t doing work to compress the air. So would you fill the bag with one breath (5 liters) at a time by displacing all the air? NO!
The trick is to do with the Bernoulli's principle: The sum of all energy (speed, pressure and height) remains constant… so what does that really mean? It means we find most efficient way and capture or harness as much energy possible.
So how does it work with story two? It’s all about position (Both your position as the user and where the business end is vs the job). [1]
Blow through the pocket bellows with the tip of the bellows a little front of an open bag (say a big bin liner)… You’ve used a single breath but the volume of air in the bag is many times more than you’ve just exhaled (like 30 or 40 liters, the liner is bigger than my small rucksack.) Again all this is without blasting full power like a professional trumpeter.
This ‘metaphor’ illustrates the technique. You can (hopefully) replicate the technique, result and you can practice.
[1] You don't need to hunch over, kneel or plant your cheek in the mud. You don't need to shovel your lips right up next to the hot embers. If you're blowing a bird's nest into flame that's maybe a little different.
Q1. Has anyone else used one of these? or anything similar to it?
A1. Yes, I have something similar.
Q2. How much difference does it make in wet conditions?
A2. I find it helps. It can’t achieve the impossible or overcome the insurmountable. But as a tool it's certainly a force multiplier.
The two best features are, it’s ease to use. You can get comfortable, and do a good job very safely. You don’t need to wave a foam mat or the lid of an ice cream tub about to fan the flames. So you won’t bash your hand or swot the stove by mistake.
The second goodie is managing the burn rate. If you’ve added a lump of Oak that’s really not burning very evenly; lance it with some air and you’ll cut through that beast pretty damn quick!
Q3. Sizing ?
A3. Diameters: 10.5 mm tapering down to 5 mm. Length: 95 mm extending to 485 mm. Weight: 25 grams: bellows, case and neck lanyard.