Answer
Direct and indirect development differ largely in the timing and extent of developmental stages. Direct development hastens some embryonic changes and produces a small adult right away without a larval stage. This could be accomplished relatively simply by changing expression of a relatively few control genes, which, in turn, alter the expression of many other genes which produce actual structures.
Work Step by Step
The concept of a few control genes altering expression of many others, called the developmental toolkit concept, is extremely powerful in explaining what was, before the 1980's-1990's, puzzling large differences among animals. These few genes are like the supervisors of an assembly line who can quickly and significantly alter what comes off the line with a few powerful instructions.
