takbeeraat-e-intiqaaliyyah

Q: At what stage should the Imaam commence reciting the Takbeer when going into ruku and sajdah? Should he commence as he changes posture or is it permissible for him to recite this Takbeer after going into ruku or sajdah? 

Q: If a person performing Salaah behind an Imaan happens to repeat the Takbeer with the Imaam, is the Salaah valid?

Q: When we move from one posture to the next posture, should we stretch the takbeer e.g. when we stand up from sajda to go to qiyaam for the next rakaat, must we pull the takbeer?