Right. Forget looking for the literature, it really isn't that big a deal so I'll just explain it better for you.
When you shift from inner to outer cog, you can shift the brifter through the whole of its arc so that you hear two clicks, which are so close together it's barely noticeable other than one click. If you are towards the top of the block (i.e. the big rear cogs) then you'll find that the front mech catches the chain. if you then trim the brifter with the small lever, the front mech moves very slightly, so that it is out of the way of the chain, but not so much so that it shifts down.
The same goes for the reverse situation.
I must emphasise again, THIS IS PERFECTLY NORMAL OPERATION, and is referred to in the shimano service instructions for the DA, Ult, 105 and I think Sora brifters these days too. Without trimming, smooth operation of the drivetrain will never happen throughout the block on one ring. Also, bear in mind that the chain should always be as straight as possible as per the shimano documentation to avoid unnecessary wear to the chain and spockets and even the front chainrings.
Hope that helps.
__________________
Matt
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