Say you have software to keep track of your grocery list. In the 80's, this software would work against a command line and some flat files on floppy disk. Then you got a UI. Then you maybe put the list in the database. Later on it maybe moved to the cloud or mobile phones or facebook integration. If you designed your code specifically around the implementation (floppy disks and command lines) you would be ill-prepared for changes. If you designed your code around the interface (manipulating a grocery list) then the implementation is free to change.

Rather than implement features you might need, you implement only the features you definitely need, but in a way that accommodates change. If you don't have this flexibility, parallel development simply isn't possible.

At the core of parallel development, however, is the notion of flexibility. You have to write your code in such a way that you can incorporate newly discovered requirements into the existing code as painlessly as possible.

Let's assume that something in the system has to change (as it rarely does..), and we need to add a new step to the process to the runner