The planes sketched
See more clearly in a sketch
This type of sketch is good to draw onto the clay, to really define the form in a sketch-like manner, before any attempt to define skin or surface. This allows the clay to be loose and fluid, but moreover easily changed, until a time that a good structure is reached and a certainty that can be relied upon to develop further.
This also encourages the
clay to be rough at this stage and refined (should you wish) as the detail is modelled. If the clay is smoothed too much, too early on, before the planes are found, there is a far higher chance of the clay getting “overworked”.