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There are three ways to set it as far as I can see from having the camera in front of me. First you have to enter C.Fn3: Drive - Restrictive drive modes - and check-mark the 14fps option.
Then you can select it as you would normally select a drive mode. It's positioned between 'silent' advance and single advance, so not beside the other continuous advance modes. And the 'H' flashes to indicate it's the 14fps mode vs. the regular high speed advance.
But that's not what you want.
Yes, it's possible to assign 14fps to a button for instant access, but the only two options are the AF-ON button or the * button. Not the M-Fn. 1 or 2 buttons. This is set through the Custom Controls CF.
Peter: I would have liked a RAW option. I don't plan to use this advance mode much, if ever, but it is audibly faster and the mirror blackout for a quick burst is surprisingly not that distracting. 1/4 to 1/3 of a second blackout will gain you 4-5 frames. Personally I would see it being more useful for remote camera applications. It's also slightly quieter because the mirror is not flapping up and down.
Also, the M-Fn.2 button is the upper of the two buttons and is actually easier, more comfortable to reach than the lower button.
Hope this helps...