Corrected entry: Just before beaming up to the Enterprise and leaving the 21st century, Picard calls the ship to give the corresponding order. He does this without touching his badge before starting to talk. When not on board of the Enterprise the badge has to be tapped to active the communications link (some sort of energy saving mode). Just starting to talk and activate a com link with this only works on board.
Christoph Galuschka
17th Sep 2007
Correction: Incorrect. There are numerous instances on the show where they keep an "open communications link" with someone on the surface of a planet. When they do this, tapping the badge is not necessary.