A natural tendency for software engineers nowadays is to divide large functional territories into digestible plots of land. Although blinded by their natural ideas of what technical activities are required to conquer the ever smaller acreage, one must avoid this urge of tasks splitting with all one's might.

Delivering a consistent and cohesive subset of all layers of a feature is better than delivering all of a single layer. By definition, a tracer bullet travels through all layers of a feature, taking with it a small piece of each layer.

For more information please have a look at Agile Estimating and Planning by Mike Cohen.