There are three things to work out.

First, you may have a special function key which determines where the video goes. The regular LCD monitor, the s-video connector or both.

Second, you need a resolution and frequency setting (/etc/X11/xorg.conf modeline entry) that your television or monitor screen will recognise. You might have the s-video output set up as an independant display in /etc/X11/xorg.conf. The video device will need to support this dualhead function. Then a program displaying on the TV will have a different screen number in the DISPLAY variable setting from the regular screen. Perhaps 127.0.0.1:0.1. ( <host>:display.screen ).
This will allow you to start a program like this: DISPLAY=172.0.0.1:0.1 <program name>
and have the video sent to the s-video output instead. You could also adjust the settings on the properties of an icon in gnome or kde so that the same thing happens. Simply add "DISPLAY=172.0.0.1:0.1 " in the command line. Note the space at the end. This will change the DISPLAY environment variable before running the command. You can instead add a X windows argument -display 172.0.0.1:0.1 to many programs. Then when you launch a program in the GUI, it will open on the TV instead of the LCD display.

Third, I have noticed that after connecting the s-video cable, you may need to powercycle the computer before the card will work. I've found this to even be the case with my Sony stereo (w/video switch) and DVD players. Going from video output to s-video output required me to power cycle my stereo and DVD player.