What does this button do?

After only 5 days I got a nice letter in my mailbox and I could get some license plates made. Yes I could drive around as much as I wanted to. Of course I took off immediatly to find the most dirty and muddy tracks in the neighboorhood of "de Wijk" (the place where I live - a small village in the south of Drenthe, near Meppel).
I was amazed where you could get with this immense machine, I bumped over rutted farm tracks, fiddling with the yellow knob and red lever. All this gearing - high / low ratio, I had to take a time out.
Luckily I found a really comprehensive manual (again on Victor Rutten's site: www.series3.nl) He spent some painstacking hours getting this whole bunch of Land Rover Ambulance manuals scanned, for which I am very greatful.
After this drive of a few hours I couldn't get this smile off my face, a thing that keeps coming back each time I get muddy in the ambu!