GP2 driver Oliver Turvey was in McLaren's Woking driver simulator after it emerged the Renault teams - including dominant Red Bull - have been allowed by the FIA to use 50 per cent throttle under braking.
Earlier, the FIA intended to limit off-throttle engine blowing to 10 per cent, but the Renault teams argued that they need at least 50 per cent on reliability grounds.
McLaren team figures said they only discovered Renault's 50 per cent concession during free practice on Friday, when Ferrari boss Stefano Domenicali was heard telling his counterpart Martin Whitmarsh "The (Red Bull) * are doing it again".
"We were saying 'Wait a second, can we do that (too)?" Jenson Button told AS newspaper.
It also emerges that Mercedes have been allowed to use 20 per cent throttle, based on the engine maps of the pre-exhaust blowing era in 2009.
it has put many teams this weekend a little bit on the back foot
"I am sure it has put many teams this weekend a little bit on the back foot so we are trying to cope with that at the moment," said Whitmarsh.
Brazilian O Estado de S.Paulo journalist Livio Oricchio reports that a further rule clarification is due in the wake of yet another technical meeting on the subject late on Friday.
"The climate of the meeting was tense," he revealed. "Today (Saturday) the final decision of the FIA should be announced."