Remember the detection point is a fair distance ( sometimes a couple of corners ) before the activation point. The car only needs to have received the permission signal before it crosses the activation point so it need not be instant. Most tracks its 2-5 seconds gap between the two points.

The cars do run very accurate gps systems tied into the data logging ( which is also used by Charlie Whiting during any post race investigations ) so you will find the car receives the permission and determines its own position and will control when it is able to be opened.

During practice and qualy sessions when it can be used anywhere some teams are running a press to open/close setup ( Brake peddle automatically closes ) where as others are running a hold to open setup. Personally I think I'd prefer to use the latter but guess it all comes down to the drivers preference and how all the other stuff is setup on the wheel.

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Stoner & Webber 2011 World Champions