Re: the stepping up section of chum kiu:
Chum kiu involves coordinated whole body motion. The stepping up is part of the footwork development process. When properly coordinated it devlops an addiitional skill of delivering power...with hands or foot.
Lots of applications can be discovered in that move.
joy chaudhuri
I think the bolded part of Joy's comment is the best answer to the OP's question, and it pretty much holds true for all the movements in the forms. Like Mook in his earlier post, I really hadn't thought much about this particular step until the question was asked. Then as I started pondering it, I began to see a lot of stuff there... and I'm sure there is a lot more that I'm not seeing. That depth is what makes WC forms so endlessly fascinating and useful. You can't reduce each movement to a simple self-defense application.