straighten your arm to its limit like you're trying to stick your hand out. Then tense and try to force it a tad further, thats how it feels. When I tried out bjj the teacher arm barred me a couple of times. It wasnt extremely painful, but it was rather uncomfortable and it felt like my shit was going to break.
If I ever competed Id tap extremely quick if caught in such a submission, hence why i dont compete in mma.