I now look at quick drawings as a type of test for my perceptive abilities.
When you draw, there is so much to think about all at once: line, shape, form, value, depth, style etc. When you do a longer, like 8-10 hour drawing, there is time to think of everything and go about it in a systematic way...one thing at a time (and even then I mess it up). But with quick drawings I usually give myself 15-20 minutes. Half an hour at the most but I usually start to screw it up at that point.
While I draw I try to think of every concept about drawing that I have ever learned. It forces me to make decisions quickly and the shapes are reactions to what I am looking at, filtering through me and about 2 seconds later there is a line or four on my paper.
I like using straight india ink and a brush (as used in the drawing above). It forces me to make bold lines and be confident about my composition. Once you make a mark in dark ink-that's it, you gotta let it stand.