Our 480s behave like that due to the clutch saver feature, one is awful as it is on lower profile tyres and wheels pins everywhere but once used to it the thing goes like stink, I only drive the units of course being class 2 
Ok slightly different but when I was tipping on thursday this is what happened
I reversed back into yard as was easier was also on weigh bridge in there yard but didnt matter as only 1 could get in at time I had opened curtains they started to tip me every so often the would do another weight then continue well got to nearly empty just had to pull curtains along other way well I was round other side 2 og the lads tried in vain to put the post back in place until I walked round put it straight in they had tried to lift the metal post straight up
They said they couldnt put it in was abut ask the folkie to lift the trailer roof as they were to embarrassed to ask how to do it not that could have been costly as some damage could have been caused they now know that system as there are a few
So never be afraid to ask it just might save you
CO-op’s Manual fleet are all like this. It’s the clutch save feature, I had a nightmare getting used to it. Like previous posters have said its about letting the clutch right out before giving it the right foot.
I hate the feature, its a pain when you are delivering to drops in say sheffield where they are all on bloody hills and you have to manouvre about a bit. Ill have the keys to the old DAF unit and burn the bloody clutch out thank you…