Is it obligatory for the descent for bowing (ruku') to be simultaneous with the takbir (proclamation of Allahu Akbar), or is it permissible to say the takbir while standing upright, and then descend for bowing?

It is recommended to synchronize the takbir (saying "Allahu Akbar") with the descent for bowing (ruku'), such that the takbir spans the entire duration of the descent. The Hanbalis hold that it is obligatory to perform the takbir during any part of the transition. So, if one pronounces the takbir while standing before bowing, it is as if they have abandoned it. However, some jurists deem it excusable due to the difficulty of meticulously adhering to it. The summary is: do not pronounce the takbir for bowing while standing; rather, begin the takbir as you start bowing and conclude it as the descent ends. The takbirat al-intiqal (transition takbirs) are a sunnah (recommended practice) and not obligatory according to the majority of scholars.