I'm really no expert, but the concept is easier. If you just move +forward you're at 320ups. Now when you activate the speedometer, you notice a slight increase above 320 when you strafe (for example, left) and turn your mouse in that direction. Thats what you want to achieve with circle jumping. You jump when you get the slight increase in speed. As far as I understand it, you start strafejumping right after the initial jump. You gain the extra speed while you're on the ground.
Oh, and I do my circle jumps with 90 degree from intended direction (like bridge2rail, hence the "practice quick turns" tip). If you're into math and theory, those two articles might interest you. Strafing theory and circle-jump theory. The speed question is answered there too, but the values are for Q3/CPM and are most likely a bit different in QL.
I'd say "pretty fast"which gives 294 degrees/sec for VQ3 and 445 degrees/sec for CPM.