Seconds earlier, Sgt Nathan Legend had instructed Rennel Walker-Arthur to kneel but had not given him enough time to respond, said Pc Adam Peters.
The senior officer is also alleged to have punched the prisoner in the face and put his boot on his head.
With his hands cuffed behind his back, Mr Walker-Arthur had not been able to break his fall and instead of hitting the crash mat as intended by the sergeant, his forehead hit the floor, causing a deep wound, the court heard.
Just before he was felled, the prisoner "had said something like 'you haven't done anything for me, so I'm not going to do anything for you'," Pc Peters told the district judge.
Asked what happened next, the constable, said: "He went down. He was taken straight forward. I think Sergeant Legend swept his legs. I heard a crack noise which I believe was his [the prisoner's] head on the concrete floor."
He said Sgt Legend, from Wolverhampton, had not protected the detainee's head as officers are taught in training when using force. "I didn't think it safe to take him to the floor in that style," he added.
The sergeant had been called from his home station of Oldbury to help out at the short-staffed Perry Barr custody block on December 22 last year when the alleged assaults took place.
It is claimed that moments before, he punched Mr Walker-Arthur three times in the face whilst he was on the floor being detained by other officers. His defence is expected to argue that the strikes were justified in the circumstances.
Pc Thomas Richardson, one of the arresting officers, said the sergeant had been "friendly, professional and courteous" when taking details of the arrest and carrying out a risk assessment at the custody desk when he was first brought in.
The detainee had been arrested in Birmingham's Broad Street shortly after 5am for allegedly assaulting a police officer and causing criminal damage to a patrol car.
At the station, he said he had been drinking champagne and just wanted to sleep.
But when Mr Walker-Arthur refused to remove a ribbon-lanyard attached to his jeans, Sgt Legend came from behind the desk and grabbed him by the neck, grappling him to the ground.
Pc Richardson said the senior officer did not follow the five-step procedure preceding the use of force, starting with a personal appeal to the detainee and getting progressively firmer.
Sgt Lee Harding, who went to see what the "commotion" was about, described seeing "a guy actively resisting the officers" but "not aggressively" before Sgt Legend landed the blows.
He said the officer had stood up and put his boot on the detainee's head. "It wasn't a stamp but it wasn't gentle," he added, rating it four out of 10 in terms of force.
Asked how he felt about what he saw, he said: "I didn't think anything. I didn't see what had happened so I didn't know what force was required."
Sgt Legend denies two charges of assault by beating.
The trial at Birmingham Magistrates' Court continues.