Rather new at this ICE in the ls650/S40.

I also had the, 'backfire'.
Riding was little problem, could be controlled.
It was Shut Down.
(Especially under the overhang at the bank drive through. And the P.O, and Cafe)

Understood the rapid, 'shut down' (riding) and the, 'big single' vacuum. Dealt with that. But the, 'shut down', "BANG".

I Believe it was, air.
Found the header bolts were loose.
And the clamp, (header to factory muffler), also loose.
Did some stuff, and reduced the, "BANG" to a 'poof'.
And if I let it idle, for about 20-30 sec, no 'poof'.

So perhaps try, 'air', leaking into the exhaust system somewhere ?
(The thing I have to constantly fiddle with is that, 'clamp', muffler over header)

So now, when the, 'Bang' comes back, a little maintenance, and it is gone again.  But in the meantime, (when I want it quiet), It's, as others  have said,  Brakes, Gear, slowly let out clutch. Or, pull the choke, then shut off. Although it probably doesn't hurt, (just like giving a dog a treat when not training, it makes, You, feel good). I don't like putting the wash of extra gas, on top of the piston.

Kris01 wrote on 10/26/15 at 15:57:05:

A word of caution on tightening the header bolts (ok, maybe a couple of words): They are pretty fragile and will break. Use a 1/4" ratchet and a light touch. You just want to snug them down, not crank them down to 100 ft/lbs.

YES,  ANY, Steel bolt, in a Aluminum hole.

The header. One was hand loose,  the other just touched the wrench.

Took it off, Cleaned the gasket, flat sanded the pipe.
(Block of wood, 400 grit W/D paper)
Anti seize on the bolts, and both sides of the gasket.
Put back in, with, ‘feel’.  Didn’t have a torque wrench that small.

Those two bolts, have not come loose.
It’s that darn ‘clamp’ between the header & muffler,
that always causes the problems.

I covered the joint in high temp silicone and then wrapped it in aluminum foil before I clamped it. So far no leaks.