Sentence:  When it <WSD>octavates</WSD>, the holes being closed, it often assumes its natural pitch again on opening the holes, instead of continuing its tones to the octave above, so that it <WSD>octavates</WSD> much more easily when the holes are closed than when they are unstopped. 
Meaning of the word marked with <WSD> tags:  To (resonate) or sound one octave higher or lower.