Point to note... you can't actually change your velocity during an interdiction. You can change the throttle position, but your speed won't actually change. It remains static for the duration of the interdiction, and "moving to blue" has no impact on your ship's agility during the interdiction. In normal, non-interdiction supercruise it will, so the key is, if you suspect you're about to be interdicted and want to try and evade it, ensure you're in the blue zone of your throttle before you get interdicted. That way you go into the interdiction with as much agility as possible.The contest is based on a ship's SC agility. Put your throttle and 75%, and stay in the blue.
I regularly have occasions I have my throttle at zero (30km/s) in supercruise and get interdicted, then pump up the throttle to the blue to stop submission, but this does nothing to change the speed/agility of your ship in supercruise. It's entirely based on what speed you were at when you get interdicted. I just verified this in-game, unfortunately booched the recording, so I'll have a video soon.