When does the ma'mum catch up with the imam in a rak'ah?

Question:
He says: Honorable Shaykh, may Allah grant you success. If the ma'mum bows after the imam has risen but before the imam says "Allahu akbar" (or the response to the takbir), has the ma'mum caught up with the imam in that rak'ah?

Answer:
No, the ma'mum does not catch up with the imam in the rak'ah unless he bends down and his hands reach his knees before the imam rises. If he bends down and his hands reach his knees before the imam rises, he has caught up with the rak'ah. But if the imam rises before that, he has not caught up with the rak'ah. Yes.