This image of the dancer Bertha Wardell was done with a brilliant light source directly behind the camera, producing the narrow, dark shadow. Probably with 3x4 Graflex/ 3 second exposure on tripod. The earlier in the series were made with 8x10 and long exposures (up to two minutes).

Weston understood lighting.The nudes of Charis at Oceano (I think mentioned in this thread) were done with morning sun directly behind him, slightly under-exposed negative.