Dave posted this
"I have all my bikes set up with 'euro style' brakes so that I can give that signal while having the main brake to hand."
As part of another thread on signalling.
Of course the left/rear brake convention (reversed in US and Europe) is due to the 'fact' that putting on the front brake (alone) was 'likely' to lead to people going over the handlebars.
This certainly can be the case with inexperienced (and light) children jamming on a brake in panic.