Here’s the more detailed explanation, it focuses on the rear end and toe, but it’s just as important front and back:
When you have an alignment done using "half" or "full" load, the machine assumes that you are carrying more load than you normally would be, so it assumes that your van is sitting lower than it normally would. The outcome of this is that it adjusts the target "green" numbers to those for a van carrying a load.
As you lower a T6 and especially on the rear end you will get toe in and negative camber. This is simply because of the geometry of the rear arms and their mounting points.
So, the machine thinks your van is sitting lower than it normally would and adjusts the target numbers to be that for a van sitting higher, it thinks that your wheels don't usually toe in as much as they are and so it doesn't ask the fitter to correct them (as much as it should). It will allow that number to go green when they are still toeing in, assuming that when you get home and take the load out of the van it will sit higher and the toe in will reduce to correct settings. What it doesn't know is that you aren't taking any load out of the van at all, so your suspension geometry doesn't change, the wheels continue to toe in too much and you ruin your rear tyres in no time!
When you have it done with "no load" settings, the machine sets the target numbers assuming that your van runs at it's current ride height and therefore suspension geometry all the time. That means it uses the correct targets for how your van is sitting at the time of the alignment. That gives a toe in target much closer to 0 and makes sure that you get the alignment you need to look after your tyres correctly. This becomes more and more critical the lower your van goes, simply because the lower you go, the further the wheels will toe in until there is no adjustment left.
