[The motor could either have no power at all, putting it in 'Freewheel' mode or could have some other mode where both inputs are high or both low... not sure if there is a difference there.
There is. If both enable inputs are off then the bridge transistors are all off and energy stored in the motor circulates current through the freewheel diodes and back to the power supply, acting as sort of a "regenerative braking" mode. Current in the motor decays quickly in this case and the motor feels "strong" braking.
If you set both sides of the motor coil to the same potential then the energy stored in the motor circulates coil through the bridge transistors only (or freewheeling diodes). Current in the motor decays more slowly and the motor feels weaker braking.
Anyway, The function table on page one of the datasheet states that if the ENable pin is low the output is high-impedance (off).
Could anyone explain what this high-impedance means and the end result, as opposed to me setting the pins so that the two wires going to the motor are both Low for example.
The high-impedance means the bridge transistors are off and current can only go through the freewheeling diodes. The end result is as I described above.
: compatible with Arduino UNO, 24V operation, all I/O's fused and protected