Hi,
There will still be power supply to the motor, to reset the power needs to turn off. Example on normal condition, the curtain is being rolled up and stop at the ceiling when the bottom bar(which is connected to the curtain) is "stuck" at the ceiling, causing an overload to stop the motor from turning further. There will still be supply there but when there is a fire alarm, the power supply is cut off and the curtain descend due to gravity of the bottom bars weight.