Examples were given throughout, by playing short passages of the music on tape. Just a little bit about the symphony itself. It consists of four movements. The inner two are about 15 mins each. I don't know whether you're at all familiar with Mahler, but he uses very complex orchestrations with every instrument having it's own voice.
Solo passages come soaring out at various points from all sorts of different instruments.
The speaker made the point that Mahler gives the second violins their own voice, rather than merely having them give depth or support to the first violins. Instead of having all the violins on the left of the conductor, with the 1st violins on the outside and the 2nd violins on the inside, he was placing the 1st violins in their usual position, but the second violins would be on the right side of the conductor. This would have the effect of separating the voices so they could be more easily heard. This moved the cellos to the center, where their sound could more easily radiate directly out at the audience. The violas were to be in their usual position to the right of center, but now just inside of the 2nd violins on the outside.