une fente qui évite le ressort de tension du fil

Some sewing machine bobbin cases have a slot on the side of the case where elastic, heavy thread or ribbon can be passed through in order to bypass the tension spring on the bobbin case. I only found one source, other than yours, which referred to it as a hole. With at least four different brands using this type of bobbin, there should be more references to a hole. Since there isn't, and given the picture at the France Machine a Coudre site given below, I think slot/fente is probably more accurate than hole/trou.

Unlike some of the other manufacturers, Bernina does not allow free downloads of its sewing machine manuals so I was not able to see if there is a specific name for this slot. Without an owner's manual for one of these bobbin cases, one cannot definitively say what the translation for this slot is.

You should use a deco thread guide for it and lower your tension and make sure that you thread your bobbin fill thru the **tension hole ** if it has one or buy a bobbin holder for embroidery if your machine manufacturer makes one.
http://www.joann.com/catalog.jhtml;$sessionid$OOQIVEAAAAWOAP4SY5LRHOR50LD3UEPO?CATID=82294&PRODID=64314

To bypass bobbin tension
1. Machines that have a **wide slot** on the bobbin case. - Bernina's 1000, 1001 and 1006 machines have a wide slot on the side of the bobbin case that you can draw your fiber through, letting it hang until you pull it up through the needle hole. This procedure applies to all makes of machines (except the Bernina 1630) which have a wide slot on the side of the bobbin case.
2. Loosening the bobbin tension spring. - All other Berninas, including the 1630, require that you loosen the bobbin tension spring with the screw on the bobbin, usually as much as it will loosen.
3. Let the thread hang out of the opening of a built in bobbin case. If your machine has no bobbin case, such as a Singer, Elna or New Home, simply plop the bobbin into the bobbin holder, let the fiber hang out and pull the fiber up through the needle hole as you would any other thread.
4. Let the fiber hang out of the top of the bobbin case. Do not put the fiber through the tension spring of your bobbin case. The fiber hangs out the top of your bobbin case until you pull the fiber up through the needle hole.
http://dollmakersjourney.net/bobbindrawing/

Husqvarna Viking Specialty Bobbin Case #920033096
Create beautiful bobbin work, also called reverse decorative stitch embroidery with the Husqvarna Viking Specialty Bobbin Case which features a **large bypass slot** for heavy threads and fine yarns or cords and an adjustable tension screw for specialty threads.
http://www.husqvarnaviking.com/us/media/SpecialtyBobbinCaseI...

Bobbin Case for CB Hook Models (1006-1530, 125-170) Chrome (school) [without slot]
http://www.berninausa.com/product_detail.jsp?PRODUCT<>prd_id...
[click on “Crochet CB” then click to see the video]
http://fr.bernina.com/product_features.asp?navigationID=6&sp...

Boîtier de canette pour le système à crochet CB [without slot]
http://fr.bernina.com/product_detail.asp?navigationID=14&spr...

Bobbin Case 9mm for Rotary Models 1630 and the artista 180 thru 185 [with slot]
http://www.berninausa.com/product_detail.jsp?PRODUCT<>prd_id...

Boîtier de canette (9 mm) pour les modèles 180 et 1630 [with slot] http://fr.bernina.com/product_detail.asp?navigationID=14&spr...

Boîtier de canette pour machines à coudre professionnelles SINGER, BROTHER, JUKI.
http://www.francemachinesacoudre.fr/s/343-Pieds-de-biche-Can...