A Brisbane pub patron has told of how a morbidly obese man ejected from a Toowong nightspot had a "blank face" and "snot hanging from his nostrils" while being restrained by three security guards accused of causing his death.

Mathew Spurr was giving evidence in the Brisbane Supreme Court trial for Royal Exchange Hotel bouncers Richard Allan Geddes James, 41, Vaughan Thomas, 29 and Dean Barry Christopher Tapin, 43, who have all pleaded not guilty to the manslaughter of Corinda man William Armando Amaya, 25.

Prosecutors allege the trio used a dangerous neck hold on Mr Amaya, who died after being kicked out of the venue following an alleged altercation on May 20, 2006.

Mr Spurr told the court he had arrived at the hotel with friends when he saw Mr Amaya being held down on the footpath by security staff.

"He was still, his eyes were closed, blank face, he had snot hanging from his nostrils," Mr Spurr testified.

"It didn't look like he was breathing."

Mr Spurr admitted he had consumed a number of beers before he arrived at the pub that night, but said he had been only a metre away from Mr Amaya when he saw a neck restraint being used.

"He was firmly gripped .. one guy I could see, his right hand was clearly across his neck," he said.

"I can remember it being around his neck, definitely."

Mr Spurr said another bouncer held Mr Amaya's arm "twisted down" while a third bouncer had hold of Mr Amaya's leg with "a firm grip."

Earlier, Crown prosecutor Todd Fuller told the jury the way Mr Amaya was restrained by the bouncers caused his death, in particular the neck restraint that could restrict blood flow to a person's brain.

Mr Fuller said it was known among security staff at the hotel that such restraints were not to be used.

The trial, before Justice James Douglas, is expected to continue for several weeks.