It explains on the site that you are to lightly purse your lips closed and buzz just like a B, hum is just another word describing the fact the sound is similar to a hum only you are buzzing. If you ever played a trumpet or similar instrument or spit something off your lips while they were tight that is similar to how you vibrate it only on a softer deeper manner.
HAIL FATHER SATAN & THE PANTHEON OF ORION!!!!