A plunge saw is a power saw specially designed for making cuts that do not reach the edge of a board or sheet of plywood. The cuts, called stopped cuts, are often used to create slots.
Many power saws, both circular and reciprocating styles, can make plunge cuts. The general technique is to drill holes at the corners of the section to be cut and "connect the dots" with a power saw. The cut is easier and safer with a reciprocating saw or jigsaw, but a circular saw cuts the straight sides more cleanly.
The available plunge saws are modifications of circular saws. A plunge saw is designed to prevent kickback, a problem encountered when making plunge cuts with a conventional circular saw. Additionally, the saw blade is very slightly canted to the direction of travel, which allows the trailing edge of the blade to spin in the kerf while the leading edge cuts.
Plunge saws come with dedicated a guide track, which allows for very straight cuts. The saw rides in the track instead of sliding along an edge, as a conventional circular saw does. The track design also makes a plunge saw a good choice for long cuts in sheet goods, such as plywood.