Custom or no, the saw is played seated, with the handle squeezed between the legs, and the far business end held with one hand. The player creates sound by bending the saw into an lopsided S shape -- the top curve is imperceptible -- and drawing a bow across the bottom curve. The note produced depends on where one draws the bow, and the shape of the curves.
It is also possible to play this instrument by tapping it with a mallet rather than drawing a bow. A particular practioner in the late twentieth century was fond of substituting his harmonica for a mallet, thus being able to say that he was "playing the saw with the harmonica". However produced, the sound made has an eerie, vocal quality.
The musical saw, though not normally struck when played, is often considered a percussion instrument. In any case, it is classified as an idiophone under the Hornbostel-Sachs system of musical instrument classification.