The structural specifications for a good sermon are comparable to the specifications by which [the Native American] fashioned his arrows. He realized that his very survival might depend on the excellence of his arrow. The shaft, therefore, must be absolutely straight, lest it wobble in flight; the point must be sharp enough to penetrate; the feathers must be in just the right amount to steady the arrow in flight; yet not to retard its flight or dull its thrust. Similarly the sermon must have a clear thought running straight through the length of it, a sharp point at the end, and just enough “feathers” to cope with the atmosphere through which it must pass on its way to the target.
I am not exactly sure what the “feathers” would be,(maybe illustrative material, maybe application?) but I find the word picture extremely helpful.