Let us first understand what a flashback is.
A flashback is an uncontrolled flame that recesses back into your oxy-fuel gas equipment, upstream of the location where the gas is designed to mix. In order to have a flashback you must have oxygen and fuel mixed together. Pure oxygen won’t flash back and pure fuel won’t flash back. You need both gases combined in a specific ratio range to have a flashback.
Check valves are designed to allow gases only to pass in one direction. If your check valves are working properly, you won’t have premixed gases upstream of your equipment’s designed gas mixing chamber.
A check valve will not stop a flashback, however, properly functioning check valves will stop back-flow of fuel gas and/or oxygen thus preventing the conditions required for a flashback to occur.