He is letting Gwen handle the business aspect, which includes contract negotiations and marketing, in her capacity as Chief Technical Officer. Rather, he fills the function of "chief engineer." It is to this specialist that new designs and modifications are submitted for approval. Along with overseeing technical development, he also reviews failure causes with engineers and approves fixes when necessary.
In essence, he oversees SpaceX's whole technical infrastructure. Having learned from the talented engineers he employed in the early days, such as Tom Mueller, he has grown increasingly qualified for that position throughout the years.