Well, the principle of extension tubes it to shift the focussing distance range closer : just for the sake of example, instead of say [1m, infinity] you'd get [0.5m, 5m] (because you're shifting the focal plane of your lens farther away of the imaging plane - sensor or film -, but I won't dwell on the optics/maths | EDIT : actually already available here on Photo.SE).
So you can put your lens closer than its "normal" minimum focussing distance, therefore you'll get a higher than 1:1 magnification.
However, there are some points to note (possible issues... or not ?) :
- depending on the length of the tube you'll lose some light in the process, around 1 stop for ~12mm, 2 stops for ~20mm.
- the depth of field is reduced also, because you're closer to your subject (if at the same aperture), getting your subject in focus might become tricky (without specific/heavy/inconvenient gear)
- the gain of minimum focussing distance effect decreases as the focal length increases (less "efficient" on 100mm than on say 50mm)
- you risk blurring because of camera movement (even very small movement at high magnification ratio, even on a tripod)
- you risk squishing your subject (if it's an insect) because the distance between your front lens and subject becomes smaller (and see Nick Miners's comment below)
- on a low quality (low price) tube you may lose AF (although it doesn't really matter for macro...), aperture because the lens contacts are not passed through
- although no additional glass is involved, using the lens to focus closer than it was intended can degrade iamge quality