A few weeks ago Van der Spek delivered a Putzmeister Pumi 28-4.89S with a 9 cbm Liebherr mixer drum on a special MAN truckchassis.

The MAN chassis is equipped with "wide spread" rear axles.
Because of Dutch regulations the distance between the rear axles (180 cms in stead of 130 cms) the customers is allowed to load more concrete.

what is the purpose of a pumi and how do they work ? I'm in the states and only ever seen pictures of them 

To much weight for our roads and high price permits.

@ pumper plucker: the advantage is that you have 2 machines in 1. A truck mixer and a concrete pump. Ideal for relatively small jobs. But you can also use a Pumi as a regular concrete pump by backing up truckmixers to the hopper.
We see them quite often in the Netherlands.

The Pumi on the picture carries 5 cbm concrete within legal Dutch road regulations.