Di2 Rear Derailleur

[HORSPWR]

I've just taken delivery of my e160 900e and on the rear derailleur is a grey plastic lever switch, it doesn't seem to feel like it does much when I flip it from one direction to the other.
Anyone know what it does and what position should it be in for out on the trails?
Cheers

 

[Cisco]

The lever activates the mech clutch which helps to keep the drive chain tensioned. This is quite important on an mtb. It's operating position is "on" which is flat on the mech.

You need to release the chain tension to remove your rear wheel. This is done by lifting the grey lever to position "off". You will see on/off positions marked on the mech body. Don't forget to return it to "on" when you have reinstalled your rear wheel....otherwise you risk dropping your chain/ripping the mech off etc. if you do any serious riding