It's for adjusting the rear suspension, clicks into the wheel on the right hand side.

 

If the 'wheel' (which adjusts the spring pre-load) doesn't want to move, even with the 'key', take the wheel off (phillips head screw) and use an 8mm socket.

 

Thanks, Riggie.

 

Question here...
I can turn the spring preload knob by hand. Does that means that without the «*key*» it just spins freely without actually adjusting anything?

 

It is turning it still. The key just gives more leverage if the adjuster is stiff/inaccessible if you had issues trying to adjust it by hand.

 

If the key is not pushed right in it can bend the plastic prongs. Keep it pushed well in. By hand is fine. Easier to count turns with a tool.

 

My tool was missing when I bought the bike. I just placed an order for a new one at the dealership a few days ago. Part # 33 53 8 546 009, $6.11 + tax.

 

The adjuster on the GT seems easier to turn by hand than the one on the ST. Additionally, on the ST it was difficult to get a full grip on the knob, due to the frame being in the way. On the GT it's easier as the adjuster seems to have been angled downward a little.