....The lower note provides the rhythm and the upper note doubles the speed.
.....Practice until, at the final speed, you play the temolos longer than you need in the piece. The best way to develop this endurance is to practice at even faster speeds. The final effect is a constant roar that you modulate in loudness up and down. The lower note provides the rhythm and the upper note doubles the speed. Then play the different successive tremolos i as indicated on the music....
Sounds the quote did not imply the context very much...ANy more help?thanks