You've got the idea. I can't think of the English off-hand, but it DOES refer to making branch connections without turning the water off.

Basically it involves a sort of clamp that is fitted to the pipe. The clamp contains a screw system and a cutter and is attached to the branch connection. When the mechanism is activated, it cuts a hole in the side of the main water pipe, underneath the clamp, so water can flow into the branch connection. It just means a disk of pipe sits inside the pipe.

You can get the same sort of system for attaching taps to existing plumbing in your house, particularly for plumbing in washing machines etc.

I have seen the clamp system called an "on-line connection collar".

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Note added at 49 mins (2007-04-14 12:03:50 GMT)
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Collier de prise (en charge) (saddle)
http://www.pamline.fr/pages/site/produit.asp?provenance=1&id...
http://www.plasson.fr/html/produit_collier-de-prise-pn-10.ht...
http://www.raguet.fr/ColliersAdduction/collier/ORD300_JDE.ht...

UNDER PRESSURE DRILLING is employed as a means of providing a branch connection to a live pipe in constant operation, where the consequences of a shutdown ...
www.alliedpipefreezing.co.uk/hot-tapping.htm

HOT TAPPING is the technique that is used to DRILL INTO A PRESSURED PIPE or vessel. The most common example of a hot tap is into a pressured pipeline. The hot tap is a means to relieve or flow pressure in a piping system, or to pump into the pressured system. The most common use of the hot tap is to relieve trapped pressure. Another use is to install a permanent plug in the tubing. When the plug is in place, one can strip tubing out of the hole under pressure and perform a hot tap under the plug, providing a means to relieve that pressure or pump into the tubing.
HOT TAPPING REQUIRES THE INSTALLATION OF A SADDLE, which makes a primary seal around the pipe or vessel to be entered. This seal is almost always mechanical, either an elastomeric or lead type. Elastomers are more common, yet some vendors use only lead seals. The saddle is usually a split assembly fastened by bolts. Welding should be avoided for vessels or piping under pressure. Welding high alloy material is difficult to manage. Under pressurized conditions, it may be impossible to assure mechanical integrity. Welding requires fuel, oxygen and an ignition source, which could possibly cause an explosion. After installing the saddle, it is recommended to install two full-bore, full-opening valves outboard of the saddle outlet. Bores of these valves should be determined by the minimum bore of the hot tap hole to be drilled.
http://www.wildwell.com/I_Services/is_ht.htm

Expansion Seal Technologies - manufacturer of HOT TAPPING TOOLS & SADDLES. Hot & wet taps for tapping, sampling & draining of pipe & pressure vessels.
www.expansionseal.com/html/stpg1.htm

File Format: PDF/Adobe Acrobat - View as HTML
the need to shut off the water supply. Lightweight, the HOT TAPPING System is hand. operated and can be easily set-up and oper-. ated by one person. ...
www.wheelerrex.com/PDF/literature/8100.pdf

Disruption of the water supply to some 20,000 people in the Abergavenny region was avoided recently thanks to specialist pipeline intervention services from Furmanite, which enabled a water main to be re-routed while flow continued uninterrupted. [...]Furmanite, the technical services company geared to maximising asset uptime, was called in by contractor Daniel Construction to undertake the necessary work while the main remained on-line and under pressure.
Furmanite applied its HOT TAPPING expertise (installing a fitting on the pipeline with an appropriately-rated valve and using an under-pressure drilling machine to cut into the operating main) to tap into the pipeline at each end of the 200 metre stretch, and tie-in a by-pass which would become the new permanent route for the line.
http://www.furmanite.com/OurCompany/NewsRoom/NewsReleaseDisp...