As noted by Stig, they work.
Living in a very very humid area, Seattle, there have been times it's been overwhelmed, but that's only been a couple times in the 10 years or so I've used them.
The insert is basically a thin and very soft sheet of plastic that seals against the inside of your faceshield. The type of plastic resists fogging, but is very soft, so not suitable to simply use as a face shield itself.
PinLock is the name of the HJC insert and system there are some generics out there, my experience with those was mixed. I think a couple other companies use a different name for basically the same thing.
The PinLock system is the one I've used along with a generic version at one point that worked OK, but gave me some double imaging at night. It was thicker than the PinLock, so that may relate to that.
The "Pin" of the PinLock references the two plastic pins that hold the insert in place on the faceshield.
The System works, but there are a couple drawbacks.
They're soft (really soft) so can be scratched badly if you're careless.
You have to be careful cleaning them. They must be clean or they'll start fogging a bit anyway.
They "wear" out. Plastic ages. As they wear they become harder and resist fog less.
You can get some double imaging, nothing fatal, but the clarity just ain't there as much at night.
Cat Crap is a waxy substance your rub in and buff out. As long as it's in play (lasts roughly 3-4 days for me in the winter here, a bit longer in summer).
There are some spray on versions that are similar, but don't work as well in my experience even if ya shake 'em up. They are quicker to apply so I have those around as well.
Keeping your faceshield up about 1/4" - 5mm helps a lot particularly at lower speeds. At higher speeds I am usually able to put it down and just use my normal breath box and lower vent.
Thanks for the input. May I ask, however: how does an insert prevent fogging? Thanks.