Physics dictates that dynamic friction is a force applied based on two parameters:
- The coefficient of friction between two given materials (determined experimentally)
- The amount of force between the two objects

The relative velocity of the two objects is not present in this equation.

In practice, it's quite possible you're generating extra heat and causing friction fade (the coefficient of friction is often temperature dependent), actually _reducing_ the amount of force/torque applied to the input shaft of the transmission.

The reason you add throttle is:
- Add energy to the flywheel
- Get the engine into a range where it can generate more torque without stalling

Thus allowing you to be more aggressive on the clutch.

Obligatory: Don't hold a car on a hill for long periods using the clutch