Sawmill is bowing each note with a separate stroke of the bow(sawing backwards and forwards, hence the name). Very common in trad cajun and English playing, more often just used for special effect by most fiddlers nowadays. The distinctive sound when a band starts "chugging" is caused by the fiddler changing from a standard shuffle bowing to sawmill. The famous lick that defines Orange Blossom Special is a special kind of sawmill bowing with syncopations.