If you're doing a lager and need to do a diacetyl rest, then when fermentation is about 75% finished, you raise the temperature to 65-68 degrees for about 48 hours
How do you know when it is 75% done?
For lagers, I've always done 2 weeks at 50 degrees, then 4 weeks at 35 degrees.
Near the end of the 2 week primary I'll warm up to room temp, for the rest, then racking.
But how do you know when the yeast has started to remove its diacetyl? As soon as the airlock stops bubbling?