There are two methods of widescreen signalling, Sky chose to use the type that is embedded in the ancilliary data in the MPEG transport stream, which is decoded by the digibox and output via Pin 8 of the SCART, a high voltage, around 12 switches the TV to 4x3, 6v switches the TV to 16x9, and 0v means switching not used via SCART.
There is also the AFD method of widescreen signalling, which inserts a code into the video, a bit like teletext, but where teletext is in vertical blanking, WSS is in the first half of line 23, which is the first line of active picture.
Some DVD recorders can record the first type using SCART switching, but all VHS and DVD recorders can record and playback the second, AFD type of signalling.
The advantage of the WSS inserted into Line 23 is that it can have more than just 4x3 or 16x9 - it actually has several modes, including forced 16x9, 16x9 with 4x3 protected. The first will force a widescreen set or digibox into 16x9, even if you've told it you want 4x3 centre-cut, the second will allow the box to do centre-cut. There are about 9 currently defined AFD codes for Line 23 WSS.