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

The questioner : Here is the rak'ah, O Shaykh.

The Shaykh : And you intended to catch up with me.

The questioner : And that one who is bowing, how does he do it, O Shaykh?

The Shaykh : As for catching up with the rak'ah.

The questioner : Bowing with his takbir. Come, you see, you see.

The Shaykh : I say that catching up with the rak'ah is by catching up with the bowing, meaning if you say the takbir for entering prayer, then you go into bowing, and you catch up with the imam before he rises from bowing, you have caught up with the rak'ah, even if you missed reciting al-Fatihah, because reciting al-Fatihah is dropped in this case, due to the hadith of Abu Bakrah (may Allah be pleased with him) that he caught up with the Messenger of Allah (peace and blessings be upon him) in bowing before he reached the row, so when he finished his prayer, the Prophet (peace and blessings be upon him) asked, "Who is this?" Abu Bakrah said, "I am, O Messenger of Allah." He said , meaning do not return for the recitation or do not return for the bowing before you reach the row, and he did not order him to repeat the prayer or to repeat the rak'ah, and this is evidence that he caught up with the rak'ah, what? Come, come.