Non-Rebreather masks - how many holes should be covered?

I'm confused and hope someone can help me out here ... I have been using hi-flow oxygen for a few weeks now (and boy does it help!) with the non-rebreather mask, made by salter labs, supplied with the O2, 

It comes with 2 sets of holes, one of which has a simple plastic cover which allows exhaled air to be expelled but closes up when I breath in.  The other is just holes, so when I breath in, air comes in and I assume, dilutes the o2.  I asked Bayer about this when I ordered my oxygen and they sent me two new masks both with only one hole covered.

I've looked at the OUCH "O2 info for patients" handout and it tells me I should use "A mask called a non-rebreathe mask which has a reservoir bag and is used with the two holes covered" 

So, should I cover up the hole or not?

Thanks