In the old days of carrying the SAA Mexican style here in the West, the loading gate was opened to give a little extra retention.

Iggy's description of using a string to retain the gun is/was popular by the street boys because when your piece needed to dumped the string provided a level of deniability that couldn't be had if you were wearing a holster.

Note to OP: If you are asking about "Mexican Carry" and your choice is a Glock or other striker fired sidearm...fagetta about it!!

Bob