you mention lots of yeast. This seems to be important across the board with lager. I have also heard that if you get enough healthy yeast a D-rest is not really needed. Is this your experience? I ask because when I do a lager I will be getting lots of yeast from the local brewery. If I pitch a quart of 1 day - 1 week old yeast slurry do I need to do a D-rest?
One reason to do the D-rest is if you can taste diacetyl. A good test is to warm a sample up to room temp and taste. The other reason is to speed the fermentation up for a shorter time length, and the increased activity will blow off more SO2.
Yes pitching a lot of healthy yeast at a temp cooler than your fermentation will reduce Diacetyl production.
I try to give a "diacetyl rest" (a ramp in temp of 2-3F after I've reached about 80% of the expected FG) to every beer, ale or lager, that's I'm not fermenting at the top end of the temp range.
It can also help clear up acetylaldehyde.
In dry-hopped beers, I'll start this rest and add the dry hops at the same time.