I've siped and grooved my own tires for years now. It does work, but
it's not a perfect cure all for everything, and can be done wrong.
Siping is the cutting of slits, not groves. It is performed with a
knife. Siping can greatly increase overall traction. Siping does very
little, if anything, for rain traction.
Siping allows the leading edge of the resulting tread block to raise up
when under load, helping to create a mechanical bond with the driving
surface. This is the same sort of mechanical bond that occurs with the
extremely soft rubber of racing tires.
The downside of this is treadblock instability. Siping produces tall
skinny treadblocks that flex and roll under load. This tends to reduce
cornering capability, especially when carried to excess. There is a
fine balance between increasing traction of a tire with large stiff
tread blocks, and creating a squirmy tire that can't corner above 20
mph.
In order to control this, you normally only sipe about 1/8"-1/4" deep
in the tread blocks. This does mean you'll be siping several times
during the life of the tire if you want to retain the effect.
You often times see stock siping in all weather tires or snow tires.
These are the little groves cut into the upper surface of the large
tread blocks.
Siping done by tire dealers is almost always nothing more then rolling
the tire over a bladed wheel. This means the sipes are cut willy-nilly,
with no regard to the existing tread pattern. As a result, the edges of
the tread blocks are cut, resulting in bits of tire that fling off and
give no traction benefit.
Grooving tires does a similar effect as siping, but also increases rain
traction. Grooving is the cutting of groves, with something like a
grinder or a soldering iron. Because the blocks are now separated by a
grove, they are even more willing to roll over. You also tend to cut
groves far deeper. This is most commonly done on offroad tires where
tread blocks are too large for the type of terrain. They are cut down
in size, increasing the number of tread blocks. This makes the tread
blocks more flexible, increasing the lip that catches traction, as well
as potentially increasing mud clearing capabilities. Done right, and
you will get much greater traction. Done wrong, and you get the most
god awful squirmy tire that flings tread blocks off.
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