These were rendered using the "light from the eye" approach in which the observed lighting is not represented - instead the light is imagined to project exactly from the eye of the viewer. The observed light and the capacity to "feel" the form in space with the eye are used to understand the form, but the form is modelled using the system - which is inherently spatial and sculptural - rather than visual. I really like to work this way, both in the case of scattered lighting or shiny surfaces (such as shiny bronze sculpture) and because it forces one to think in terms of planes and helps to avoid a more passive copying of visual appearance.
The men pulling ropes are studies for a painting I am composing, thinking about what poses certain figures can be in, getting to know the types of things people do in this action, investigating their anatomy.
The other image is an imaginary form, loosely based on some photographs of ink being dropped into water. Of course, in this case, there was no light to observe in the first place, since it was imaginary, so an intuitive system for applying light and shade consistently is really helpful.