Zero power at zero cadence

It is not logical to have power value when I stop pedalling. Even the smart trainer may be still generating power due to inertia. It is not difficult to detect cadence. It may be an option.  At the moment the power value gears downs slowly with 3 second average and with instant power. 
At the moment one has to wait for a while to activate the superduck position during descents.  
One would like to sprint heavily and then go to superduck immediately. It takes a long time to wait for the power value to drop to zero from 1000 watts while already not pedalling.