The PTO opening is the approximate 4" x 6" steel cover on the side od the transmission. It will have 6 bolts attaching it. The PTO itself is installed onto the side of the tranny where this plate is located and is shifted into and out of gear by either cable or lever. The old ones (40's-50's) were nearly all lever controlled but nowadays the cable is generally used or all that is available for your old tranny. A shaft is attached via a U-joint to the PTO output shaft and it attaches on the opposite end to a winch, pump, sprocket or whatever to operate a piece of power driven equipment, i.e, a dump hoist as red 58 mentioned. However it is also possible to use a quick disconnect shaft on the end of the driven shaft so it is just like a farm tractor. The PTO will have a percent of engine speed ratio from 40%-120% depending on model and type. Direction is important depending on what is being driven.
