I regularly (like every morning) go up a hill that is loose rock standing up. They are right, it is about shifting your weight, and keeping the stroke smooth.
You can put just as much weight over your rear tire standing as sitting - unless you have invented anti-gravity and aren't telling us - but, you do have to make sure you don't lean too far ahead. The biggest issue is that you can really hammer the pedals - which causes you to break free. Work at smoothness and you can do it. I start off slower and slowly increase speed.
Clutchy, if you are slipping on every stroke, it indicates that your stroke is uneven - otherwise you would just keep slipping.
just my dos colones