Note that all throttled spark engines have a variable spark voltage, but it's the pressure in the cylinder that determines this voltage, not the ECU.
When the ECU decides it's time for a spark it cuts off current to the coil primary (for an inductive ignition system), and the voltage on the secondary of the coil naturally rises until the gas between the spark plug electrodes is ionised. The voltage required to achieve ionisation is roughly proportional to the pressure in the cylinder (
Paschen's Law), and this pressure increases with engine load, i.e. when you open the throttle, more air is allowed into the cylinder and the coil voltage has to rise higher for the spark to form.