|There's more than one way to do things|
In both the traditional model and in XP, you might be thinking ahead strategically. They part company when it comes to tactics. The traditional model requires what XP calls Big Design Up Front, which involves planning out design and construction well ahead of time. XP reduces tactical scope by restricting design and implementation to one "feature" (user story) at a time.
In the traditional model, you (try to) avoid coding conflicts by planning ahead. In XP, you avoid conflicts by taking one step at a time, having a full set of regression tests so that you prove on short notice that your system is in a working state, and refactoring before you're done with the step.
There's more parallelism in the traditional model, with testing deferred until the system is once again back into a working state. XP tests all the way along, keeping the system in a working state between short steps.
Clarification: In the traditional model, the phases of development happen in serial, while implementation activities happen in parallel. In XP, phases happen more in parallel, with implementation happening more in serial.
Thanks to tilly for indirectly pointing the need for a clarification.
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