The flame can flashback and burn inside the torch, or even back up and into the hoses to the tanks, flash back arrestors (check valves), (backflow prevention valves), are devices that let gas flow one way only, these are very important they will protect you from an explosion.
If you are using a torch and begin to hear a stand hissing howling noise, shut off the fuel, as the torch is burning on the inside of the torch, (I will usually cool down the torch with water also, wet rag or splashing water on the outside of the torch), the flash back arrestors are normally connected behind the torch handle, between hose and torch, even if you have them, there is a possibility they could fail open on old equipment, so shutting off fuel is important when a flash back’s occur.
Getting a torch too hot, a dirty torch, faulty equipment, improper use can cause these flashbacks (burning inside the torch) to occur.
Checking the flashback arrestors and other equipment (regulators, hoses and fittings, tanks), especially on an old torch or equipment is important, keeping the torch clean, and proper maintenance and keeping the torch (system in good working order is important) not only to have a tool in good working order but also for your safety.