Where others saw a smile on Corporal Kok Yuen Chin's face, his rota mate and fellow full-time national serviceman (NSF) Mohamed Rabik Atham Ansari, 22, saw fear.

The lance corporal said he knew his fellow Singapore Civil Defence Force (SCDF) officer well enough to tell the difference between his "smiles".

And when the SCDF servicemen around them started saying "kolam, kolam" during a cake-cutting ceremony for Cpl Kok, Lance Cpl Mohamed Rabik said he saw the former give a "scared smile". The "kolam" ritual involved an officer being made to climb into the 12m-deep pump well at Tuas View Fire Station.

Lance Cpl Mohamed Rabik said he and Cpl Kok were close and often together at the station. He was testifying yesterday on the third day of the trial of Lieutenant Kenneth Chong Chee Boon, 38, and First Senior Warrant Officer Nazhan Mohamed Nazi, 40.

The officers face charges over the death of Cpl Kok, 22, who drowned on May 13 last year during a ragging incident that saw him being pushed into the fire station's pump well, also known as kolam in Malay, as part of activities to mark the completion of his national service.

On the night of the incident, Chong was rota commander and Nazhan, deputy rota commander.

They are each charged with aiding a rash act that caused grievous hurt by illegal omission. They allegedly failed to prevent a group of officers from making Cpl Kok enter the well, thereby endangering his life.

Lance Cpl Mohamed Rabik said the "kolam" ritual was brought up during the celebration to mark Cpl Kok's impending operationally ready date. He noticed his friend was "scared, but smiling" and when the celebrations ended, Cpl Kok "rushed" to leave the control room.

In Lance Cpl Mohamed Rabik's account, "kolam, kolam" was shouted in the room where most of the officers on duty, including Chong and Nazhan, had gathered. Chong's defence lawyer Wee Pan Lee asked him to clarify what he meant by shouting, and the SCDF officer said the words were spoken "louder than normal", but noted that many were talking and the room was noisy.

Lance Cpl Mohamed Rabik said four officers, including himself, carried a "resisting" Cpl Kok towards the well located across the yard. He remembered Cpl Kok "wriggling" his body "mildly" and interpreted this as resistance. Mr Wee suggested that it was possible Chong did not hear anyone mention "kolam" as the room was noisy, and that whatever went on in the yard outside happened without his client's involvement.

"(Chong) worked with the men long enough to know he could trust them and not cause harm to each other," said Mr Wee. "He also knew that if Cpl Kok did not want to do something, he knew that the men in Rota 3 would not force him to do it, do you accept that?"

Lance Cpl Mohamed Rabik said: "I accept that, we were all a family."

Nazhan's defence lawyer Singa Retnam also asked him if he heard Nazhan telling Cpl Kok during the celebrations: "If anyone 'kacau' (bothers) you, tell me".

Lance Cpl Mohamed Rabik said that while he remembered Cpl Kok standing near Nazhan, he was not aware of such an exchange.

In a police statement after the incident, Chong said he knew the group was taking Cpl Kok to the well only when he saw them gathered there, and acknowledged he failed in his duty as a supervisor to stop the act.

The trial continues today.