Power planers are to jack planes as portable circular saws are to handsaws. Both the planer and the circular saw are powerful electric-powered tools; they do much the same work that the jack plane and handsaw do, or once did, but they do it more quickly, sometimes more efficiently and accurately, and always at a higher decibel level.
The power planer is a hand-held tool, but it operates like an upside-down stationary jointer. There's a cutter-head with a pair of sharp knives that, like a plane iron, removes shavings of stock. The cutterhead is aligned with the rear portion of the tool's base; the front shoe of the plane adjusts to control the depth of cut.
The power planer cuts no more than a sixteenth of an inch at a pass. The depth of cut is adjusted on most models by a control knob mounted atop the front of the planer. Some power planers come equipped with an adjustable fence.
The size of the portable power planer is determined by the tool's maximum cutting width. Most models available on the market today plane a maximum width of between three and a quarter and six and a half inches.
Operating a power planer is similar to using a bench plane, but requires much less effort: You don't need to drive the plane, rather, you guide it along the path you wish planed. Clamp the workpiece securely and make sure your stance is balanced. Although little force will be required, use both hands to control the tool, with your left hand guiding the plane at the front, the right balancing the rear.
Before planing the length of a board, put some pressure at the front of the plane to ensure that the sole is sitting flush to the piece (rather than on an incline, with the toe lifted above the piece). Likewise, be sure the heel is parallel to the board at the end of the planing stroke, just as you would with a bench plane. This prevents dipping, the defect that occurs when more wood is planed from the ends than from the center of the stock. Check your work for flatness as you proceed, using a metal straightedge.
The power planer has many uses in a workshop, particularly a smaller workshop that doesn't have a full-sized jointer-planer or surface planer. It will remove sawmarks, even off rough edges, and trim stock with ease. It's invaluable when fitting doors, either the full-sized or cabinet variety. It'll taper and surface, too.
A final word of warning is in order, however. Like the bigger jointer-planers, which are known for shaving off fingertips as well as stock, this machine has a wicked bite. Use it with care, planning, and respect.