Is it allowed to follow the imam at the same time

I see alot of people praying in the mosque in congregation, they always go into rukoo' at the same time of the imam when he is about to say *Allaahu akbar*, but then, they go into rukoo' when he is about to say the first part *Allaa-* or do whatever he is about to do, and i feel like this is wrong, even though he they should go after he finishes saying it and ***goes first*** is it allowed to do this?