Quite accidentally I've stumbled upon the phenomenal method of quick gestural sketching demonstrated by Stan Prokopenko.
I can't find enough words of praise for the man and his relaxed 'it's easy to be done, just go with the flow' approach to understanding how a human figure functions in an illusional space, being dynamic and NOT rigid even in 'passive poses'.
Go for it and - above all - enjoy.
You won't be sorry. Like, ever!