July 21 defendant 'defused bomb'.
Wearing bright purple gloves, Manfo Kwaku Asiedu showed a Woolwich Crown Court jury how he opened the door, put his hand inside and pulled out the battery.
He said he then removed a tub and poured the hydrogen peroxide and flour mixture which it contained down the toilet.
"I opened it slowly," he claimed. "When I saw the battery what I did was I put my hand (in) and I pulled the battery."
Returning to the witness box after the demonstration, he continued: "The yellow stuff - the flour that was mixed with the hydrogen - it was all ... on the floor of the (side)board. It was bubbling.
"I took it and I went to the toilet and I poured it in the toilet."
Several weeks ago "the fifth bomber", Ghanaian-born Asiedu - who allegedly lost his nerve at the last minute - "broke ranks" and turned on co-defendant Muktar Said Ibrahim, accusing him of planning an attack "bigger and better" than July 7.
He claims Ibrahim rigged up the sideboard at the north London "bomb factory" to explode.
Asiedu (34) of no fixed address, admits helping to buy dozens of litres of hydrogen peroxide but claims he did not know what they were for.
He and Ibrahim (29) of Stoke Newington, north London, are among six men accused of taking part in an extremist Muslim plot to carry out a series of murderous suicide bombings on the London transport system.
The others are Yassin Omar (26), Osman (28), Ramzi Mohammed (25) and Adel Yahya (24).
They all deny charges of conspiracy to murder and conspiracy to cause explosions likely to endanger life.
Asiedu told the nine-woman, three-man jury that on the evening of July 21 he returned to 58 Curtis House - the alleged bomb factory -to sleep.
He had earlier dumped his rucksack device in Little Wormwood Scrubs, a park not far from the prison.
Asiedu said the next day he remembered he had seen Ibrahim kneeling down by the sideboard on the morning of the alleged attempted attacks.
Asiedu said he went over to the sideboard: "I twisted the key and I started pulling, just slightly, and I saw another tub.
"I stood back and I started thinking, I said: 'It's the same device, it's been left here. Why was it left here?'."
Asiedu said he then opened the sideboard again and removed the battery and tub.
Asiedu said he left the New Southgate flat taking the detonator from the device with him, and "chucked" it away.