Blowing main drain question

[ian81]

Hey just a quick question, closing real soon here in michigan.. My 3rd year closing but I am confused on how blowing my main drain keeps water out of it? I never really thought about it, just followed instructions from the previous home owner, but anyways..

I use a shop vac and when bubbles come flying out of the drain I then close the valve and turn the vac off.
How does the water not go back down thru the drain a little?
I'm A bit tired and at work so my apologies if I'm not making sense.

 

[Bama Rambler]

Actually a little water does get back into it, but it doesn't matter since whatever gets back into it never gets high enough to reach the frost line so it won't freeze. Even in MI your frost line isn't 8 feet deep.

 

[JamesW]

It's like when you blow into a straw and then hold your finger over the top. The straw stays full of air.

 

[ian81]

Of course! That makes sense, thanks for clearing that up!