It lets air out, but not in. So you don’t have hot air coming in and raising your machine temperature above where it can operate.
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It’s a sliding door that fits in a frame that you connect two hoses to. When you want to use it you slide it open. When you’re not using the machine you slide it closed and no air can make it through the hose.
I made a “Check blast gate” sign for my GF that I hang on the blast gate when it’s open but stick on top of my GF when I close it. The first time you run a job with the gate closed you’ll know why 
Here’s what they look like. Someone also posted a file here to make your own but they’re cheap already made.
Where can I get one of these? Or can I see a photo of yours?
I don’t have one, but a lot of people have posted about them, so you can probably find some sources in the forums.
EDIT: @jamesdhatch just gave you a link to one up above. 
What you described is a back draft prevention damper which opens when air is flowing, a blast gate has to be opened and closed.
The damper won’t work on our systems because the airflow is too low (even with an inline 4" fan".
Damper:
They make louvered covers for dryer vents that open with very little airflow, that might work.
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Well technically she’s correct. A blast gate is manual but does let air out but not in. The backdraft damper is both automatic & one-way - it’s a special case of a blast gate
But that’s why I posted my reply - it needed more detail for someone who would want one but didn’t know what one was 
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Very confused by this. An open blast gate provides no restriction to airflow in either direction. At least the one I have doesn’t. If left open, it would be no different than not having it there at all.
It lets air out when open and does not let air in when closed.
It’s non-directional but it does exactly what she said.
A backdraft damper has the advantage of being automatic and directional.
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@jamesdhatch posted a photo-link to a blast gate. eflyguy posted a picture of a back draft prevention damper. The Louvered Vent Cap is a popular option and is found at Home Depot, Lowes, Menards, etc… as it is meant for the end of a dryer vent (which is also a four inch hose.) Here is a picture:
You’ll find that picture in @marmak3261’s post about his DIY magnetic quick disconnect coupler.
There seems to be a rumor that you can buy one, but I didn’t find any links to a commercial one in my very quick search.
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I have the one that lets it out - no in
This one?
MagVent MV-90 Magnetic Dryer Vent Coupling
https://www.amazon.com/dp/B013TI7A4S/ref=cm_sw_r_cp_apa_i_rMyADbZ874A3T
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So I am freezing my fingers off to see if the temperature lowering would make a difference.
There is a vent under the table where it sits as well…so It might need a afghan too.
If I could find a klondike bar it probably wouldn’t even need the freezer@
No bueno
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Has support been in touch since you sent the picture they asked for?
No, I think I missed a picture that they needed. I sent that about 6 hours ago.
They are sending another machine
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#4???
I am really sorry about you having to go through this all over again…and especially sorry about your plans for making stuff for the wedding. I can only imagine how utterly disappointed you must feel.
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Thank you…your so kind.
I feel like I’m whining all the time. lol
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You are incredibly patient with this whole thing. I hope the next one lasts for YEARS.
Not at all…and even if you were, you deserve to whine a little and all of us here are so sympathetic to your problems. I hope the wedding is perfectly wonderful, even without your creative additions.