What does "short brake" mean?

It is "short-circuit" -- the Control Function table on page 4 says that you can make it actively drive both outputs to the same level by either driving both inputs HIGH, or driving one input HIGH and the other LOW while commanding the PWM low. This shorts the motor out via the power or ground rail.

You should be able to do this continuously -- the main limit is the temperature of the motor windings and FETs as that's what dissipates the brake energy if something is trying to spin the rotor at the time.

Short brake is when the rotation speed creates back EMF in kV/RPM and is short circuited by a shunt FET across windings with suitable logic control on inputs Hi or Lo i.e. both to V+ or both to V- or both to 0V in bipolar supplies.

No power is applied to load at this time.

However I once had to stop a servo/circular sin/cos pot in 1ms without re-design... so my solution was short circuit the bipolar supply to the miniature motor for 1ms directly between V+ and V- to motor to Gnd thus creating a momentary short to 0V across motor coils stopping it instantly without excess heat. This was all done using serial SCADA commands to servo and it worked perfectly.

But in your case with only single supply. It can safely shunt to V+ or V-(0V) without supply drain.

The trick is to know the inertial mass on motor and BEMF and compute I* ESR*t energy absorbed during fast brake then Energy pulse width vs SOA of component for margin.

If you had a large inertial mass like a flywheel, one could burn out the Driver FET or BJT unless you pulse the Brake like PWM to regulate the Fast braking current. Of course this also means you need PWM to soft start or use VFM method with V/f pseudo sine wave methods in other applications.