How do they do it without reference?

[McLean]

just a question.... so when bengal draws those girls from many different kind of perspective without any ref, its based on his strong foundation on human anatomy and perspective?

I think its safe to say "yes"

Its certainly not his ability to cook or do algebra that allows him to create those drawings.

[Farvus]

Long time ago just like in Nickillus' case I assumed that imagination drawings are done completely from imagination. I had some single book on anatomy/perspective and learned from that but what got me started was just accepting the fact that my drawings will always be far from realism. It was sort of like climbing the mountain with too big rock on my back. I didn't realise I had it and even though it prevented me from reaching the top, I grew some imagination drawing "muscles". With a little bit of additional observation I can run twice as fast though .