The wrist bands act upon a point known in chinese medicine as Pericardium 6.
If you look at the inside of your wrist, you'll see two creases. From the centre of the lower (nearer to your elbow) crease, measure down the length of the front joint of your thumb. That shows you where P6 is, if you feel around that part of the wrist, you should identify that this is a slightly more tender spot than other nearby parts.
The band is applying variable pressure (basically a stud that wiggles around a bit as you move) to this point, which [for reasons even chinese medical practitioners seem rather vague about] quells nausea.
You don't need the band - if you aren't flying, massage it gently with a finger or thumb from the other hand, and it has the same effect.
There's a diagram of the Pericardium Meridian and its points
here
G
(Hobby interest in the practice of aspects of oriental medicine)
N.B. If buying sick-bags, spend a few extra pennies and get ones that seal closed!