The 400 is the old eiger re-named a King Quad - different tranny.

This is for the KQ 450/700/750. And probably the 500, or is it 550? Anyways, the CVT tranny KingQuads with hi-low range.

Just right.

Here's a youtube vid of it.
www.youtube.com/watch?v=koI9MqtqXQw

The best way to think of it might be that you need to listen to the motor. When you pop it out of gear into neutral, wait for the revs to drop to half of what they were and then pop it into high range. (high range is half the gearing down that low range is) It is very forgiving of a slight mis-match and it goes right in smoothly unless you are way off.

If you are going slightly uphill then don't try it until you are familiar with the fact that the ATV is slowing down while you are in neutral as well as the engine slowing down. You have to wait longer in neutral to accommodate this.
If you are going up any more than a slight grade, forget it. Wait until it levels off or stop and shift.

On the other hand if you are going downhill then you hardly have to wait at all because the quad speeds up as the motor slows down.
Get used to it and do it. It's great.
I don't even give it a thought anymore, it's all automatic to me.
Like a truck driver gets used to licking through the gears without the clutch, just by listening to the engine.
ps. I have class one license too. hahaha

Hey, if you hear some chattering while trying this because you didn't do it quite right, don't worry at all. It's not gears grinding, it is the dogs on the sides of the gears looking for compatible speeds, and those things can take so much abuse you would be amazed. You aren't hurting anything. I kid you not. It's just noise.
The actual gear teeth are always engaged. The gears slide to and fro to engage the dogs but the drive teeth never disengage. That's why it's called a 'constant mesh' transmission. Those gears don't grind.
But in no time at all you won't make any noise anyway, if you ever did.

Cool trick, can't wait to give it a try...

P.S. Nice work on your king quad glenlivet, did you built the receiver plate on the rear? and also is that HMMW skid stuff easy to get a hold of like local hardware store or something? is it like puck board? I heard people starting to use puck boad for their skid plates as it slides nice and is strong...

Yes the receiver plate is home made. The skid is faced with UHMW sometimes called puck board. You have to go to an industrial plastics supplier for it. If you are making the skid plate of it alone you need to use at least 1/4" stuff. It bends with the aid of a good hair dryer or heat gun.
I used a bending brake with my 1/8" stuff, cold.