Understanding Chunking Twos

I came across this concept of "only focusing on or counting the downstroke" for each pair of up and down stroke picking in the pick slanting seminar. I could not fully grasp this concept back then but was able to picking pretty fast and thus stopped ponder overing it. Now, I am watching the Synchronicity lessons and hearing this concept again, I realize that I need to work on it. My biggest struggle is that I do not understand how to just focus on the downstroke when down and up strokes are performed and how to know or feel if I am doing it right. I tried my best to not put effort in the up stroke when I shot the "4-note chunking" video. In order to place more focus on this, I even tried to do some double picking, which is a term I heard from other players, by playing a note twice (i.e. with a down stroke and an up stroke). When I do this, I could not play as fast as I was in the "4-note chunking" video and I was sounding very sloppy as well. May I know how should I think about this thing so that I can focus on the down stroke only and clean up my double picking. Thanks!