I've posted this before, but I'm not sure I'm describing it very well...
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The extra strap is to stabilise the bag when it's worn 'messenger bag style'. It's hard to explain, but I'll try 
This is easiest to do the first time if you (temporarily) remove the padded bit from the main strap. Unclip the connector on the small strap then slide it round near the adjuster of the main strap (i.e. the opposite end of the main strap to how you've shown it in your pics).
Put the neck strap over your head (with the bag on your right hand side), then swing the bag round right behind you. Now slide the small strap up about as far as the middle of your chest. You should now be able to pull the connector down and to the left, round behind you to connect with the clip near the bottom of the bag.
I hope that makes sense 