"Once a month or so, Jimmy (1999 World Judo Champion, Jimmy Pedro, Jr.) gives a clinic for an hour at the club. Usually I'm not on the mat, but this time I walked out there for some reason and a little girl pipes up and asks me to help her. I think it was sankaku (a triangle choke) they were learning. Anyway, my first reaction was to think she was doing it wrong because it looked different from all of the other kids but then I caught myself and realized, wait a minute, she was just doing it to the opposite side. That's when I got to thinking about the fact that we almost always do matwork on the opponent's strong side.
Look out on the mat and you'll see that 90% of the time, the person is arm barring their opponent's right arm. That doesn't really make sense does it? From now on, what I'm going to start having people do is to go for the weaker arm.
I kind of sort of agree with Jim. If you are going to only train one side, it probably makes more sense to do the opposite of what everyone else is doing because your opponent won't expect it. It also makes more sense to attack the weaker arm. It is also true, as I looked through just random pictures I had of me doing juji gatame that it was always the person's right arm.
"There are no sides in matwork. You're not standing up. It's like in swimming, no one does a left-sided backstroke. "