Cyril B. Aug 3, 2013:

? Quite the contrary, then. Opening the fuel line – with the petcock – is a logical step of the start up procedure.
Isn't that "button" also involved in the shutdown procedure? Like, closing the fuel line?

Kévin Bernier (asker) Aug 2, 2013:

I do not have the chinese text. Also, the button is involved in the start up procedure of the machine, which rules out your suggestion.

Cyril B. Aug 2, 2013:

Chinois ? Si ton texte source en CN est sélectionnable, pourrais-tu poster le terme source ? Ca pourrait aider.

Alison MacG Aug 1, 2013:

In that case ... it could be a mangled translation of half-throttle lock/switch/button (see component lists of user manuals in EN)

or perhaps even "reverse switch" (if you put your bouton à demi-tours into Google translate, you get "button turnarounds")
Example from China:
Brush cutter
Reverse switch for grass clipping removal
http://www.globalsources.com/gsol/I/Brush-cutter/a/900000012...

It seems strange that the manual lists a component without offering any further details about what the component actually does. If the customer cannot help, it would appear we can only wish you good luck with this one!

Kévin Bernier (asker) Aug 1, 2013:

P.S: The french text may not be completely accurate, as it is a translation from Chinese to French. Sadly, the only help I have regarding the document is the source document with the images.

Kévin Bernier (asker) Aug 1, 2013:

Images are provided with the manual. It is a brush cutter which uses cutting wires, so perhaps this button has something to do with the inclination of the wires?

philgoddard Aug 1, 2013:

Yes, or it could be something to do with half turns or U turns. Can the customer help?

nweatherdon Aug 1, 2013:

This kudoz question is the only Google hit.

I wonder if it could be a button that sets it to half speed or something. Not a clue on a correct technical term at the moment.

Good luck!