The first reciprocating saws were built in the early 1950's by Milwaukee Tools, carrying the name of Sawzall. The name obviously stuck with the tool, as Milwaukee's Sawzall has been a name that has been synonymous with the reciprocating saw among builders for many years.
Modern reciprocating saws have quite a few features that were not found on some of the original Sawzalls back in the 1950's. For instance, modern reciprocating saws are almost all equipped with variable speed motors. The speed at which the blade reciprocates back and forth is controlled by how far the trigger is depressed. Simply put, squeeze the trigger a bit harder for more speed. This easily adjustable speed mechanism allows the operator a great deal of control, particularly helpful when attempting to make some wide curved cuts (such as moving from the side of a window cut-out to the sill on the bottom of the window cut-out).
Another widely adopted feature on today's reciprocating saws is an oscillating blade action. This oscillating motion means that for every back stroke, the blade angles downward into the cut slightly, and for every push, or forward stroke, the blade angles upward slightly. This causes an oscillation motion that helps cut through materials, particularly wood, much faster. It tends to cause a rougher cut than a straight back-and-forth cutting motion, but reciprocating saws typically aren't considered precision cutting tools, so the finish of the cut is rarely an issue.
One feature that has been adapted in just the past few years is a chuck that allows for tool-less blade changes. By twisting the chuck with one hand, the blade can be slid out of the chuck with the other hand, and a new blade inserted quickly and easily. Most tool-less chucks can even allow for the blade to be inserted upside-down (with the teeth facing upwards) for added cutting versatility.
Reciprocating Saw Blades:
Reciprocating saw blades typically are sold in two different lengths, a short blade (about six inches long) and a longer version (about 12-inches of exposed teeth). While the longer blades can obviously cut much deeper, they are harder to handle, can whip side to side quite easily, and are also very prone to bending, particularly when the tip is forced directly into another object.
Reciprocating Saw Safety:
Speaking of long blades, keep in mind that at high speeds, longer (and thinner) blades will have a tendency to whip side-to-side. When you encounter this whipping action, ease off on the trigger to slow down the speed of the motor, which will help to bring the blade back under control. This is especially problematic when blades are a bit used, as the considerable heat (generated by friction between the fast back-and-forth motion of the blade and the edges of the workpiece being cut) can weaken the steel in the blade, making it more susceptible to whipping.
Of course, since this friction generates quite a bit of heat, the blade will be difficult to handle when removing it from the chuck. The best way to handle a hot blade is to simply hold the saw vertically, loosen the tool-less chuck and let the blade fall onto a safe surface below where it can cool safely before being picked up.