The root cause of kickback is the workpiece getting pinched between the
blade and the fence. There are a couple of reasons why this happens:
The most common reason is that the workpiece turns slightly (i.e.
doesn't stay tight against the fence) while pushing it through. Keep
in mind that the teeth on the blade are slightly wider than the rest of
the "meat" of the blade. This means that during an ideal cut, the only
parts of the blade ever to contact the workpiece are the teeth. In
reality, however, if you're not careful to constantly push the
workpiece up against the fence, it can turn a little, and the front
corner can come into contact with the inner part of the blade. The
friction between the blade and the workpiece causes a force which wants
to further turn the workpiece which causes the workpiece to bind even
harder, causing more friction which causes even more turning and
binding until, a fraction of a second later, the piece comes flying
back at you at mach 10. Inevitably, despite rotating rapidly through
the air, it will strike you in the gut with its sharpest point every
This scenario happens almost exclusively when you are cutting something
which is wider than it is long. It's also the reason why you should
never use the miter gauge at the same time as the fence, unless you're
VERY careful to apply pressure to keep the piece between the blade and
the fence pushed tightly against the fence or against the miter gauge.
This also happens fairly frequently when cutting very large pieces of
plywood. Despite your best efforts to keep the piece held tightly
against the fence, it's way too easy to torque the workpiece and bind
it against the blade. On large sheets of plywood, once it binds on the
blade, it usually rides up on top of the blade where it gets hurled at
you at high velocity, leaving a nice rustic scarring pattern (and
possibly some blood stains) across the face of the plywood.
Note that in opposition to Brian's advice, ripping a 1" strip off a 24"
piece is LESS likely to cause binding (and, hence, kickback) by keeping
only 1" between the blade and the fence. The larger the ratio of
length:width of the piece between the blade and the fence, the better.
Another common reason for kickback is improper use of a pushstick. If
you're ripping a piece 2" or wider, you can pretty safely push the
piece through with your fingers. When the piece is narrower than that,
however, you'll want to use a pushstick or scrap of wood to push it
through without getting your fingers too close to the blade. If you
don't use a pushstick and simply guide the piece along from outside the
blade, the part between the blade and the fence will most likely be
shot backwards once you finish the cut. This is a lot less violent
than the other scenario, however. Depending on it's length, the piece
usually only flies 5 or 10 feet.
Kickback can also occur when cutting warped boards. Sometimes even
straight boards will warp once you cut them and release the tension
from the grain. This warping can sometimes cause the piece to push
away from the fence and bind into the blade. In most cases where
warping is an issue, you'll be talking about ripping boards that are at
least a couple of feet long. That usually means that the length:width
ratio is high, so you're not that likely to experience kickback. You
may, however, experience enough binding to burn the wood or even bring
the blade to a screeching halt. This can be a dangerous situation, as
you don't want to remove you hands from the workpiece allowing the
blade to start back up and throw it backwards, but you also need to
shut the power off as soon as possible. Often, the circuit breaker on
the motor or in your breaker panel will trip in this case. A splitter
can help immensely to avoid this problem.
You can also experience kickback (and rough looking cuts) when the
blade isn't perfectly parallel to the fence. You'll want to make sure
your saw is adjusted to keep the blade, fence, and miter slot all
parallel to each other.
Last, if you try to cut pieces that are exremely small, especially
without a zero clearance insert, you're almost guaranteed that the
piece will kick back at you. The good news it that the pieces are
light enough not to break any ribs when they hit you. The bad news is
that such pieces are sometimes small enough to penetrate the skin
rather than just bruise. They also tend to ricochet in random
directions adding to the fun and sport.