Here’s a video from two-time U.S. Olympian Nick Del Popolo on how to get inside the defensive stiff-arm using the “chicken-flap” method.
The other side of this is, don’t be a defensive stiff-armer! In randori all it does is stifle the development of both you and your partner. In shiai the stiff arm can be applied momentarily when needed, just for a second. Keep your upper frame relaxed and ready. If you need to stiffen your arms in defense, you do it just for a second, then you relax again.
If you’ve ever done any kicking or punching, whether it’s boxing or tae kwon do or what have you, you know what I’m talking about. What’s the best way to generate power in a punch? Do you flex all the muscles in your arm first? No, that’s ridiculous. You have to keep your body relaxed almost all the way through. Your arm is like a whip – it’s only tense for a millisecond at the moment of impact. Power comes from correct technique, not from having big muscles.
Same thing with judo. Obviously there is some muscular strength involved with all your movements, from brushing your teeth to doing a push-up. The idea with judo is to be as efficient as possible – seiryoku zenyo – and staying relaxed is a big part of that.
(As an aside, there’s nothing wrong with having big muscles, as long as technique and relaxation are primary. Muscular strength applied through correct technique is bananas! But if you have big muscles it’s very tempting to rely on them, particularly during the complex lifting and turning motions required in a judo throw. This is why we say the best judo throws feel effortless.)