Here is an Edgar Payne. I am going to use the sky in this painting to talk about thrusting forms. A form is a shape in a painting and by thrust I mean its capacity to suggest directional movement. A form that is round has no thrust, a form that is arrow shaped has a lot. Artists often use thrusting forms to carry the viewers eye about the canvas, or to counteract or interact with other forms in a design. You are probably familiar with this concept in lines, knowing that a line may convey the viewers eye in a direction, well, a form can do the same thing. Look at the picture below with cryptic lines and numbers on it.
Number 1 is a cloud that points inward toward the center of the painting. Number 2 is another. Number 3 is a sky hole that carries the viewer around the turn. That is , when you look at it the semicircular shape carries the viewer through that part of the painting. Above 3 the cloud points inward and to the right. Number 4 is a big inward, and downward pointing shape. I think you probably get the idea. Almost every shape in that sky is designed to be directional. Would you go back to the top and unaltered picture and look at the sky, see how many different thrusting cloud forms there are and their direction of thrust?
This idea can be used in fields, architecture, surf, whatever is in your painting. This is one of the keys to getting vitality in your shapes and controlling the viewers course through the painting.
Here is another thing I want to point out. I have numbered the blue sky holes in the painting. This is a great example of variety of shape. Each of these numbered shapes is as different as can be from each of the others. They vary in area and in shape. Number 1 is for instance, shaped nothing like number 5. This was not observed, or copied from a photograph, but designed. It is an abstract arrangement that has particular and deliberate qualities that the artist thought would look cool.