Tyre Inflators

[interele]

Why do tyre inflators have a setting for flat tyres. Yesterday I had a flat tyre to inflate and since I was there I decided to do all the other tyres. So I set the pressure to the correct setting and did the other tyres. Then I went to the flat tyre and it wouldn't do anything. Setting it to 'flat' let me inflate it OK. How does that work ? Is it that the 'back pressure' allows it to gauge the pressure or is it that the valve doesn't shut unless there is pressure is in the tyre ?
Ta

Mal

[barry1010]

I think you are right, if there is not enough pressure in the tyre then the machine won't automatically start. The 'flat' setting is just a manual start button really