It's the AlKo ATC system which automatically applies the van brakes if it detects imbalance. I ran two 540's without it and never needed it. One van I had it on I only felt it work once, when the nearside wheel dropped into a deep pothole. A momentary brake application from the rear. I'd say it wasn't necessary but if you load badly and don't drive smoothly it may stop a sway developing. And at present there are no ATC units available as they're made in Ukraine.