D'apres ce que j'ai lu sur le site d'ultimarc, c'est le moyen d'avoir une image lisible et de n'e pas endommager ton écran lorsque le PC balnace du 31khz.
"What about half-screen mode?
This is a feature of the J-PAC which allows a stable picture to be displayed from a PC running in 31Khz (VGA) mode using a normal VGA card (as opposed to our ArcadeVGA card) on a 15Khz arcade monitor. What actually happens is, as the horizontal sync frequency is divided by two, there are two pictures displayed on the screen side-by-side. This is NOT meant for serious PC application use! It is useful for checking all is well with the boot process. It is usually possible to type in DOS sucessfully for configuration changes etc. A Windows picture (or rather two pictures!) can also be displayed for limited use. Some monitors/VGA cards display this mode better than others."
Si tu n'as pas d'arcadevga, tu dois utiliser powerstrip pour avoir du 15khs
