Braking by short circuiting possible?
Yes, it is possible to do what you are asking (remove power and then short across the motor to cause a braking effect). The short allows high current to flow creating a magnetic field that opposes the rotor's field and produces the braking effect.

Of course, you must design it well otherwise it will be dangerous.

1) If you simply apply it at speed, the braking effect is sudden and can cause the rider to be thrown off, or mechanical damage.

2) Accidentally applying the shorting switch while power is on will result in short circuit, and at the levels you are talking about (400 Watts), fire.

The best way to design is to either custom design the BLDC controller or work with a manufacturer or find a BLDC that implements braking.

The BLDC controller already has MOSFETs electronically controlled by logic circuits and a microcontroller than can be sequenced to short the motor windings, so no additional parts are needed; just proper logic and firmware in the controller.

The BLDC controller can provide proportional braking control, not just ON/OFF.

Also, the BLDC controller should provide heatsinking, current monitoring, and temperature monitoring. Braking current can be very high and the BLDC needs to make sure that its own current and operating temperature limits are not exceeded.

So basically, look into a BLDC controller that can do braking if you want a good design, or use a shorting switch if you want something possibly dangerous and unsafe.