Kickback is a real concern whenever ripping boards on a table saw or a radial arm saw where the cutting head has been rotated 90-degrees for ripping along the fence. In either case, the saw should be equipped with safety features that can help make the operation of the saw safer for the user.
Of course, having safety features built into the saw doesn't preclude the operator from following the rules of safety for using the tool, which typically include but are not limited to wearing safety glasses, using hearing protection when warranted, wearing appropriate clothing, keeping the hands a safe distance from the saw blade or cutting head, and standing in a position that will reduce the risk of injury in the event of a problem.
Kickback occurs when the two sections of a board being ripped on the table saw or radial-arm saw move toward one another after being cut, pinching the saw blade. When this pinch is enough to clamp onto the saw blade, the spinning motion of the blade propels the board back toward the direction from which it is being fed, usually with considerable force. Kickback can be unpredictable and quite dangerous, which is why tool manufacturers have worked to come up with safety features that help to minimize the risk. A riving knife is a critical piece to reduce kickback, as this piece of metal helps to keep the two halves of the board from pinching together after the wood clears the saw blade.
Anti-kickback pawls are another innovation designed to help reduce kickback. These pawls consist of a pair spring-loaded metal grabs with aggressive teeth on one edge. The pawls are mounted just aft of the riving knife and are designed so that when the board passes the riving knife, the teeth glide over the top edge of the board on either side of the cut. As the board continues to be pushed through the cut, the two halves of the cut board merely pass along underneath the pawls but should the motion be reversed, as would occur in a kickback, the pawls are designed so that the teeth would dig into the wood, preventing the board from being propelled back toward the operator.
In most basic ripping applications, the pawls I've used on table saws and radial arm saws seem to work well, without too much of a hindrance to the forward motion of the cut. However, there are times that you may not be able to use the pawls. For instance, just like a riving knife, you can only use them on through cuts, which would render them useless when using a stacked dado blade to cut dadoes or rabbets.
One thing to keep in mind when using anti-kickback pawls is that they prevent reversing of the board as it is being ripped. So, if you only need to make a partial rip on a board up to a certain length, you wouldn't be able to back the board away from the saw blade, as the teeth on the pawls would engage and prevent the reverse motion.
Another issue that may occur with anti-kickback pawls is that the teeth on the bottom of the pawls may cut somewhat into the surface of the board being ripped, even if the teeth of the pawls don't engage and dig into the wood in the event of a reverse motion during the cut. As such, sanding or other surface preparation may be required on the boards before they can be used in a project, particularly if the woods are especially soft or prone to surface damage. While this may be a minor concern, it is certainly one to keep in mind.
With all of that in mind, in most cases, anti-kickback pawls are a useful safety feature, one that should be used whenever possible.