The "grip" and the thumb
La pince, c'est la forme de la main gauche autour du manche en train de plaquer un accord. Le pouce est appuyé sur l'arrière du manche tandis que les autres doigts appuient sur les cases. D'une façon générale, le pouce est appuyé assez haut sur le manche, dépasse pour étouffer la corde de MI grave si nécessaire, et parfois il est même utilisé pour appuyer la note basse d'un accord sur la corde de MI grave. La pince en manouche est assez différente de la pince dans The "grip" (I don't know the English word for that) is the shape of the left hand around the neck when fretting a chord. The thumb puts pressure behind the neck while the other fingers push on the frets. Generally speaking, the thumb is positioned pretty high on the neck, and is often used to mute the lower E string, and sometimes it is used to fret a bass note on the lower E string. The grip in Manouche is somewhat different from the grip in other guitar styles where barré chords are used and where the thumb rests in the middle of the neck. Pictures are better to describe this.
A few shots of a chord where the thumb is used to fret a note (G6/9 here)
A shot of a chord fretted without the thumb with the root on the A string (C6/9 here). The grip here is not so different from that on the previous picture excepted that overall the hand is less in tension and the thumb is high on the neck.
A few shots of another chord fretted without the thumb but with the root on the lower E string this time (Gm6 here). The thumb is a bit less high on the neck than previously, and a bit more shifted on the left compared to the fretting fingers.
Finally, a shot of the grip when some high notes are fretted above the 15th fret.