It is well known that a table saw's main role is to rip wood. Table saws were designed as tool to rip wood and that is what they are best doing, and that is what most of them are used for most of the time. The type of material you are cutting can make a big difference to your safety and the ease with which you cut wood, and to the actual wood you are cutting.
For example, if you are cutting material like plywood, MDF, hardboard, chipboard or any of the other “manufactured woods and materials” splitters or riving knives are not as critical, though they are still useful.
If on the other hand you are ripping natural woods like oak, maple, fir, pine, cherry, beech etc, for ripping these kinds of wood, whether kiln dried, green or in between, it is critical that your saw be fitted with a splitter or riving knife. If your saw does not have a splitter or riving knife you could be risking serious injury and poorer cuts and here's why.
Many of the natural woods woods have built in “tensions”. The word “tension” within a natural wood simply means that these woods, when they are cut, they can often move on their own as they pass through the blade. This is because the blade is releasing these build in tensions. You can't always tell if wood has a built in tension or not. Some woods are worse than other, broad leaf maple for example tends to have more tensions. These tension develop in the tree as it is growing and are magnified by branches, tree twisting, tree growing at an angle or growing on a hill and trying to grow upright. All of these features can create tensions in the wood as the tree grows.
Anyone who has ripped or even cross cut wood will remember some cuts seem to want to bind the blade, sometimes they are so strong they will actually stop some less powerful table saws. This is where the real danger comes in. The power of even a small table saw is such that it can hurl a piece of wood back at you in an instant with enough force that can cause a significant injury.
Even if the wood that has a tension is pushed through the saw, the problem can sometime create a poor, rough cut, resulting in another cut having to be made, which may be fine if you have enough wood, if not, now you have ruined a good piece of wood.
Splitters and riving knives do pretty much exactly the same thing, they help to prevent or eliminate the problem of the wood binding against the saw blade and / or the saw fence. The result of this is a much safer cut which in turn also creates a better cut.
If your saw does not have a riving knife or a splitter you can easily install one. They are available from a number of places, including Lee Valley, Rockler or even your local dealer. Click the Rocker link at the top of the page and type in splitter into the search box. The catch with installing a splitter is that the throat plate on your tablesaw may not easily accept one. The solution for this is easy, you can purchase material to make your own throat plate (and in the process you will likely make a better one that came with your saw). They are not difficult to make and the WILL make a huge difference to your ripping once you have it installed with a splitter.