In that link, why is the guy advising to turn the ignition to the on spot before disconnecting the battery?

 

Honestly, having worked 26 years at a gasoline storage facility, that sounds like a real setup for a huge fireball. You are injecting oxygen (air) into a hazardous atmosphere (gasoline tank) under pressure. One wrong spark from anywhere and you would definitely get it all out.
In answer to the question of why you need to disconnect the battery with the ignition on, that is in reference to a Softail Convertible with a siren alarm system. That is the only way to disconnect the battery and not have the alarm go bonkers! That's all. The rest, I believe, should apply to any other softail as far as the procedure. I chose attacking the crossover hose from the left side of the bike as there is just more room to get your hands and tools in that area than on the right.

 

Pick up a siphon tube (the kind with a squeeze bulb),a section of fuel line and some hose clamps. All are pretty cheap.

Siphon as much gas as you can out of the tank. Then attach a section of fuel line over the the end of the siphon tube. Cover the motor and floor with some rags. You will usually have to cut off the stock hose clamp from the cross over line the first time you remove the tank.

Have a pair of vise grips or some sort of plug ready. What I try to do is pull the cross over line off with one hand and put my thumb over the end while at the same time putting the siphon tube/fuel line over the metal nipple with my other hand. This should minimize the spillage.

The remaining fuel in the tank can drain out of the siphon tube. Then I can plug or crimp the cross over line. I prefer to plug the line with a small wooden dowel rod cut like a pencil. I'm less likely to scratch anything with a piece of wood. I leave the siphon tube attached while removing the tank to catch any left over gas.

I replaced the cut off hose clamp with a spring type clamp and have had no leakage.

 

I put a "T" in my line with a small section of hose and a plug. I pinch the tail section and remove the plug and install a longer section of hose with a connector. Run the hose to my gas can and I am in business with no mess!

 

Absolutely what he said.... cause that's what I do....