Here is what it looks like.
I have to admit that it did seem like a superfluous tool to me at the start, but I've come to love it, especially for lower lash application. To use it you gently nestle the tool up against the skin you're trying to 'guard' and paint away.
Also a tip for those who are new to it - don't try and use it for the first time during a busy morning where you don't have time for mistakes, I wound up stabbing myself in the eye with my mascara, and completely ruining the make up job I had already done! And I leave mascara for last ...
You can pick one of these up really easily from eBay by typing in "Mascara guard" I bought mine from the guys here (they sell a lot of bathroom and beauty stuff)
Do you guys have any quirky make up tools that you swear by? I want to hear about it!