Hello I just watched this video (http://www.youtube.com/watch?v=JjUPYB8x-Cc) but I can’t work out what happens between 1.00 and 1.09. It seems that he goes from a high poly mesh to a low poly mesh really fast and I don’t get how he has done this without doing retopology. It also doesn’t look like he’s used any decimate tools because of the clean geometry. I’m new to Maya and Zbrush as I ussually use Blender, maybe I’m just being stupid and missing something obvious? 
Thanks in advance for any answers 
Fristly, this is a demo so not all the steps are being shown, its designed to be sort of a show off of the process.
Looks like he started out with a low poly object, with optimized topology, this is not an uncommon thing. He subdivided it in zbrush and added the desire details, when exporting it back out to bake it was either exported as is in high res, or decimated down, he could have exported the low res and high res out of zbrush as well, zbrush offers a couple of ways to do this, goz, subtool master, being two of them.
Also if you want to try an automated method of reducing polygons I would not suggest maya, download the free meshlab program and use the quadric edge collapse decimation function under the filters, in my experience it works 10 fold better then maya’s decimate.