I've been advised to pitch an alcohol tolerant yeast "at high krausen" into a barley wine where the fermentation has slowed (OG 1.101, now 1.035). But if I've got it on a stir plate, how do I know when high krausen is? I suppose I could turn-off the stir plate every so often and let it sit for some time and see how thick the krausen gets. But I didn't know if that was a good idea and what those intervals should be.
It's hard to know when the yeast has hit high krausen, since the constant stirring prevents a krausen from forming, but with a starter, in many ways, you don't really need to know....
The idea behind pitching at high krausen is to pitch actively fermenting yeast. With good yeast stock, after about 18-24h your starter will be actively fermenting, and will have gone through several reproduction cycles (3-8 hours each), so the cell count will have been bumped up and viable cells will be in solution. You can pitch, knowing that viability is good and that the yeast are actively fermenting, even though you don't have any visible signs of fermentation. A quick test to show that the yeast have been active is to take the flask off the stirplate and swirl it - you should see lots of CO2 bubbles coming out of solution.
Before pitching, you may want to turn off the stirplate and leave the yeast to settle for 30-45 mins. Then you might see the start of a krausen ring form. Also, the dead or inactive yeast cells and trub will sink to the bottom, leaving the active yeast in suspension in the wort, which you pitch, leaving the trub and lazy yeast behind. (Note that this is opposite to when you let the starter ferment out - then you decant and discard the solution, leaving just the yeast slurry at the bottom. We don't do that here since we want only the most vital and viable cells.)