Well I cannot answer how it burns back up into the torch handle and back up into a pressurized line (without the aid of added oxygen to the fuel), but I do know it will, I have never had a tank, or regulator explode, (I keep good backflow prevention valves on my torches), but I have had it burn back up into the torch many times (normally when getting the torch too hot, when this happens I normally shut off fuel supply cool the torch head and clean the torch.
I also check the operation and condition of my back flow prevention devices, every so often to make sure they are in good working order.
This is the purpose of the back flow prevention check valve on your lines, to prevent the gas from burning back up into the torch towards the tank or regulators.
Also for those using oxygen on their torches, never use oil or grease on any parts of the torch, tank regulator or fittings, this can also cause an explosion, also do not use plumbers pipe dope to stop leaks, it contains oils and can cause an explosion.
http://www.google.com/#hl=en&tbo=d&rlz=1R2RNQN_enUS457&spell=1&q=purpose+of+backflow+prevention+device+on+acetylene+oxygen+torch&sa=X&psj=1&ei=mw8SUfXtBenLigKPvIHADA&ved=0CCwQvwUoAA&bav=on.2,or.r_gc.r_pw.&fp=abfef61ff81de054&biw=1024&bih=539
http://www.google.com/#sclient=psy-ab&hl=en&tbo=d&rlz=1R2RNQN_enUS457&q=safety+precautions+using+oxygen+acetylene+torch&oq=safety+precautions+using+oxygen+acetylene+torch&gs_l=serp.12...0.0.2.6532.0.0.0.0.0.0.0.0..0.0.les%3B..0.0...1c..2.serp.hmGDIKWENIE&psj=1&bav=on.2,or.r_gc.r_pw.&fp=abfef61ff81de054&biw=1024&bih=539
info from Wiki:
Non-return valveAcetylene is not just flammable, in certain conditions it is also an explosive. Although it has an upper flammability limit in air of 81%,[5] acetylene's explosive decomposition behaviour makes this irrelevant. If a detonation wave enters the acetylene tank, the tank will be blown apart by the decomposition. Ordinary check valves that normally prevent back flow can not stop a detonation wave as they are not capable of closing before the wave passes around the gate, and for that reason a flashback arrestor is needed. It is designed to operate before the detonation wave makes it from the hose side to the supply side.
Between the regulator and hose, and ideally between hose and torch on both oxygen and fuel lines, a flashback arrestor and/or non-return valve (check valve) should be installed to prevent flame or oxygen-fuel mixture being pushed back into either cylinder and damaging the equipment or making a cylinder explode.
European practice is to fit flashback arrestors at the regulator and check valves at the torch. US practice is to fit both at the regulator.
The flashback arrestor (not to be confused with a check valve) prevents the shock waves from downstream coming back up the hoses and entering the cylinder (possibly rupturing it), as there are quantities of fuel/oxygen mixtures inside parts of the equipment (specifically within the mixer and blowpipe/nozzle) that may explode if the equipment is incorrectly shut down; and acetylene decomposes at excessive pressures or temperatures. The flashback arrestor will remain switched off until someone resets it, in case the pressure wave created a leak downstream of the arrestor.
Check valve A check valve lets gas flow in one direction only. Not to be confused with a flashback arrestor, a check valve is not designed to block a shock wave. The pressure wave could occur while the ball is so far from the inlet that the pressure wave gets past before the ball reaches its off position. A check valve is usually a chamber containing a ball that is pressed against one end by a spring: gas flow one way pushes the ball out of the way, and no flow or flow the other way lets the spring push the ball into the inlet, blocking it.