A lot of it is to do with respect. So giving the child warning that it's soon going to be time to go home. Explaining why you need to go home. "Validating" the child, making it clear that you understand where they are coming from. Acknowledging the child's feelings.
It's not a cure all. No parenting ethos is going to result in a 100% peaceful life with no tantrums or arguements. It's just how you deal with them when they arise.
The same goes for punishment. UP doesn't ban punishment, by any means. But teaches you to question the point of it. If, say, Billy scribbles on the wall - our first instinct would be to punish Billy. But UP teaches you to question why Billy scribbled on the wall. Was it attention seeking? Was testing limits? Was is just that he didn't have access to pens and paper?
If it was attention seeking - surely as parents, it's our responsibility to ensure our children have enough of our attention. Even if it feels to us that they get plenty, they may not. If it was testing limits, then it wasn't necessarily malicious. It's obviously something to explain to the child was wrong. If it's boredom/not having access to pens/paper, then that's easily rectified.
It's a bit of a simplistic scenario, but I hope you see what I'm getting at...?
I appreciate that a lot of people will grow up feeling that they were loved unconditionally, even without UP parents. But a lot don't.
I guess it's akin to the different ways of weaning. Some puree-wean, some BLW. All children will become adults who eat solid food. But the journey they take to that point, can be just as important. Similarly, some puree-weaned kids will eat everything you put in front of them, and some BLW'd kids will be as fussy as hell.
BLW doesn't profess to be a cure-all, 100% successful system. But it suits some people, and fits in with their lives beautifully. Equally, others don't get on with it.
UP is similar. Even down to the way it's misunderstood by some!