This is the "soft start" facility doing its stuff. Should the unit subsequently click off and fail to do anything, the most likely cause (Powrwheel tell me) is a dodgy battery and they also tell me they have had more failure reports from vans fitted with 110 amp.hour batteries that those with 80 or 90, despite the apparent advantage of fitting 110AH units, particularly on a heavy van.
I understand that without soft start, the van can sit there and then move very suddenly, whereas with Powrtouch the movement is more gradual - just like me, really.