At acc altitude when you SET ALT on ADU you get Alt Green, the ptw becomes inactive until VS or IAS is selected again.
so incase of engine failiure you set Alt at acc alt then accelerate to white bug and simply set IAS on adu once your at white bug, the speed captured by the ias mode will be the white bug speed (if IAS is pressed on reaching WB )
PS - Next time you have Alt green Try using the PTW