One of the recipes I am following requires half of my dry hops to be added when the krausen is high. I always assumed that adding dry hops then would mean that there is loss in aroma. I was wondering if someone could explain the rational for adding them at high krausen. What is it that I am trying to do here?
The rational for adding hops at anytime during the ferment is an attempt to get some biotransformation. There is some evidence that active yeast will transform certain hop oils onto new aroma molecules that wouldn't have been there in the original hop make up. This is very yeast strain dependent and hop variety dependent. There is much research still being done on this phenomenon.
So splitting the dry hop charge between traditional dry hop post ferment and some during active ferment is an attempt to get the best of both worlds. It's a frequent technique for IPA brewers these days.
To address your concern about fermentation scrubbing out aroma; I wouldn't be too concerned about it. Yes you can smell hops during the fermentation which would indicated you are losing some aroma. But it shouldn't be treated like all the aroma is escaping, its a small percentage in my experience. Furthermore, to overcome that loss you'd just add more hops really to balance it out. But it's not something that is easily quantified, its an personal taste/perception correction. I've never found it to be a problem in modern day IPA brewing. We are bombing the beer with hops to a point where some scrubbing is not noticeable.