Complex sentence: Pressing the octave key will open either the top tone hole in the neck of the saxophone, or, whenever the G key is fingered, the top tone hole will close and a small tone hole will be opened near the top of the body.
Split into simpler sentences: Pressing the octave key opens the top tone hole in the neck of the saxophone. Alternatively, whenever the G key is fingered, the top tone hole closes and a small tone hole is opened near the top of the body.