Most numerology numbers look outward at the clock — the day you happened to arrive. The soul urge number looks inward, at the name you were given, and specifically at the part of that name you can't shorten or consonant-cluster away: its vowels.
The logic, as numerology tradition explains it, is that vowels are what a name actually sounds like when spoken slowly — the part that lingers — and so they're treated as carrying something closer to a person's private motivation than the fuller name, which is said to reflect how a person presents to the world.
Whether that's a meaningful distinction or simply a convenient way to generate a second number from the same input is a fair question, and one this page doesn't try to settle. What can be shown plainly is the arithmetic — and that's where we'll start.