It is certainly necessary to break down most stories into bits of work to be done. Scrum calls these bits “tasks”. We used to refer to tasks in XP as well. Problem is, if these are technical tasks, things often go less well than they might. We imagine a smooth set of tasks, but we wind up with too much work on one task, too little on another, and both too much on one end and too little on the other end of a third. Integration is a nightmare, and what we get doesn’t turn out so smooth. It looks like this:
It seems this happens much of the time. A better way? Split along story lines, not task lines.