What does the upgrade to the threat tracer do? I am confused. ...

Hope this helps

Thanks. Where did you get that from?

http://www.amazon.co.uk/Crysis-Official-Strategy-Guide-Brady/dp/0744012449/ref=sr_1_1?ie=UTF8&s=books&qid=1301909471&sr=8-1

The sole reviewer of this guide on Amazon is not an easily pleased chap. This is one of the best guides I have read.

Like bullets you can see where grenade are coming from. Where they were thrown and angle of arc.