Inconsistent detection of the sync edge results in lines not being aligned properly. This is called jitter and straight vertical features become jagged. When the Free Run option is off, Pasokon TV follows the average sync locations to minimize the image degradation from missing and corrupted sync signals. However there is still some pixel jitter. With the Free Run option on, a precise crystal oscillator is used instead for timing and the exact sync position is ignored once locked onto the signal.
Pixel jitter is reduced and image quality improved.
There is one disadvantage. If the transmitting station is not calibrated properly, the image will be slanted. New users of the low-end systems often transmit a few slanted images but other helpful hams get them straightened out. Unless you need to compensate for signals with improper timing, leave the Free Run option on for best results.