Yellow air line?

It doesn't "raise" the axle it just dumps it to facilitate turning.

Here's a picture of the pigtail in question.

 

It dumps the air to the rear axle only. Its used to relive some of the weight on the rear axle to cut down tire scrub such as when making tight turns at low speeds in parking lots. It also makes the trailer behave like its 38' instead of 48' because it will pivot on the front axle.

 

Bingo. That pigtail is kind of hard to find on the road. It has an inverted pin to prevent guys from mixing up the electrical connections on the trailer.

 

Mine gets used on almost every turn I make.

 

Just gotta be sure no cops are around, dump it, and go on. (to the op, dumping the air while on a road could land one a ticket for being overweight.)

 

I can't back up for the life of me with mine dumped. Only for the tightest of spots do I dump mine. I do believe I've seen tmcs with the front axle lifted as well. Or it could have been prime.

 

The yellow cord is used to power up the abs circuit. I just had to rewire mine to match my truck to a more modern trailer. Iirc the inverted plug is the ground and the centre plug is switched power so the abs circuit will function properly. A tradition cord uses the centre pin for an aux power. Thats where 99% of mechanics would run a dump valve/lift axle/intr light etc off of.