so basically a pull up resistor allows electricity to flow (above a recognisable voltage)
The current flow through a pullup resistor will be (should be) negligible, and the function of a resistor isn't voltage dependent, in the sense you are describing. Any time there is a potential difference across the resistor, current will flow. In the context of a pullup resistor, it's almost tempting to describe that current as a side-effect, except of course that with no current flow, there would be no voltage drop at all, and thus we wouldn't be able to use a pullup to bring a pin to a potential of 5V (or whatever our logic HIGH is).
Here's another description of pull up/down resistors which I found to be quite helpful.http://www.thebox.myzen.co.uk/Tutorial/Inputs.html
(That's by forum member Grumpy Mike)