Dog flap?

Hi
How does this dog flap thing work then?
How do you keep your house secure and have a dog flap?
Ive always wanted one but just presumed some undesirable would crawl in through it and empty the house! (while being watched by over friendly labs)

With English Setters I've always had to have the huge ones and yes, a person could easily crawl through them. My only answer to that one is that an undesirable may be put off venturing inside the house having seen the size of the dog flap and not knowing what type of dog was in the house. Then, if they were brave enough to crawl through, they would have been met by in excess of 10 Setters who would love nothing better than to pin them down and slobber all over them...a quick escape would be impossible

I have a dog flap that came with its own "security flap" that you can slot in place over the flap when you go out.
I never used to use it until dogs started being pinched from gardens where I used to live.
Our flap was only 9x11 and the house was alarmed - so if anyone could get in that gap, if what they wanted to pinch was larger, they'd have to open a door or window and the alarm would go off