synchromesh gears allow you to shift even if the input and output shafts are rotating at a different speed, thus negating the need to double clutch(which is matching the input and output shaft speeds before engaging into a gear)
from dodgy memory and trying to simplify it, the input shaft and output shaft meet before they engage, so as to match speeds
i'll try find a picture for you as i cant properly articulate myself for some reason :/