How do they work? I mean, what do they actually do to overcome human scent?
Not sure anyone could ever measure that.
Personally, I've had the best results by just using good scent control methods. Outside of hunting the wind, and as far as products go, I just use the hair and body "green" soap in the shower and Sport Wash for my clothes. I do my best to keep my gear, including boots, scent free as well by using the green liquid soap on my Summit climber (seat, storage bags, and rail padding. For a ground blind, I set it up in the driveway before the season and use the green liquid soap, a bristle brush, and water hose to scrub it clean, then let it air dry and get it in the woods soonest. I guess you could probably not clean the blind if you get it in the woods several months before the season allowing the human scent to dissipate.
Sometimes, I'll use a scent-killer type spray but that's about it. I refuse to get into many of the other gimmicks like Nose Jammer, Ozonics, etc, etc.
NOTHING trumps hunting the wind, but with rising thermals, you can get away with a little more during a morning hunt than you would with an evening hunt with falling thermals. YMMV.
ETA: Here's a video I took in Kansas of a buck that came in behind me right at day break (= no thermals) and COMPLETELY down wind. I really believe that this is a great example of what good scent control techniques can do for you. He's in my scent stream for almost a minute and a half. I am sitting about six feet off the ground in a short ladder stand (my buddy has vertigo and cannot do the heights) and completely exposed.
I apologize for the crappy video skills, but turn the volume up and you can hear him grunt at the 5 second mark.