Overview: How Does a Synchronizer Work?
The synchronizer’s job is to bring the next gear (up shift or down shift) up to speed so the shaft and the gears are the same speed. The synchro has few parts. The synchro hub (splined to the shaft) and synchro ring (slides over the hub in either direction) is held in place by the keys/retaining spring. The outer ring slides over and first engages the blocker ring (this has friction material that aids the gear up to speed) and then the engagement teeth on the gear.
This is a comparison of T56 synchro on the left and TR6060 on the right.
The outer ring of the synchro slides back and forth.
This is a synchro sandwiched between two gears.
This is a side of the gear that faces the synchro. If you look closely, you can see the little engagement teeth just above the main gear teeth.