if you scroll down in that link, you can see an animated GIF of the armbar. i've also attached a picture of the kimura.
The kimura looks scary cause it's pretty far back. the problem though was the angle. it's hard to tell from the picture though, but i remember from watching that the elbow was at an obtuse angle. to get the proper tap, you need it to be about at about 90 degrees, but i think 80 actually is better. when the arm is straighter, a lot of guys can muscle out of the kimura or just resist it with brute strength.
i found these pictures on some forums. in the process, i noticed that there's a billion threads on the matter right now. i was giving GSP the benefit of the doubt that he was being nice, but a lot of people are just straight up saying he was sloppy.
if you look at the pic, his knees are splayed way wide because his feet are crossed. now a basic rule you learn early on is never to cross your feet while you do an armbar because it relieves pressure from the arm, gives slack to the shoulder so that it can take a stretch, and removes weight from the other guy's body so that he can move around/escape.
now this is a great rule early on to follow, but later on there definitely are times to cross your legs while doing an arm bar. it's very situational when you need to, and i can't really describe when; it's more a matter of feeling. the truth of the matter though is that when you do cross your legs during an armbar, 9 times out of 10 it's just to hold the position, not to finish. most all the time when you finish the armbar, your legs should be straight. if possible, i like to flair my feet out as i pinch my knees together (kinda pigeon toeing). this gives an extra little grab at the arm, but its not often that you can get that based on your proportions and the other guy's.
the last bit i'll say is that he was "playing the slot machine." when you take a good armbar, you glue the guy's wrist to your chest before you sit back. you never let the arm come off your chest as you go down. a bad technique is to sit back, then pull the guys wrist to your chest to finish, like you're pulling the lever on a slot machine. if you test this out and do it right, you'll notice that the guy taps way before your reach the ground if you keep his arm glued tight to your chest. this is because as you drop, your torso naturally levitates up a bit. this causes your hips to float up and pressure his arm very quickly.
a side reason that playing the slot machine is bad is because you'll more frequently have the problem of pulling his arm into your junk. some guys wear a cup because they think that solves the problem when really the armbar was just bad. if you pinch your knees tight and get the arm tight before you go down, then it won't slide down into your junk.
on a side note, i hate when dudes purposefully pressure their cups into your arm or whatever to cause pain. this normally causes the "....oh really..." moment before you have to turn it up on the guy.