I took Taekwondo for a good 9 years (3-4 days a week) and earned my 2nd degree black belt just before the dojo closed down due to an expired lease and the owners couldn't keep up with the rent. My instructor incorporated A LOT of Akido in his lessons. I really loved the Akido, it was a lot of fun. If you can excel in Akido, you can win almost any fight. The problem with Akido is that it deals mostly with joint manipulation, and I'm pretty sure some of it is not allowed in the UFC. They have a rule against "small joint manipulation" which I'm assuming means fingers and maybe even wrists. But I don't see why the armbars and throws I learned would be not allowed. Anyway, on with how to do a standing armbar the way I know how.
Now the standing armbar can usually only be done when someone is punching you. The only way you can fail at not tapping someone out with this move, is if your opponent is significantly larger than you, significantly stronger than you, or is extremely tough, or is extremely agressive and relentless with his attack. We will assume that the opponent is punching with his right hand.
Step 1.) When the (right hand remember) punch is about halfway to you, the first thing to do is get ready to block. Take your left arm and hit the inside of your opponents right arm hard enough to almost deflect it. (Basically a strong backfist) A good thing to do is punch to the face at the same time with your other hand to throw them off a bit.
Step 2.) After the punch has been deflected, do not pull your blocking (left) hand back yet. After you actually hit your opponents arm, you want to follow through and make a big "C" shape. Always making sure that you're bringing his arm around THE OUTSIDE of your body. In other words, if you're using your left hand, you will be moving your opponents arm counterclockwise, and if you're using your right hand, then it will be clockwise. Either way it's ALWAYS around the OUTSIDE of your body. Follow through with the backfist, and at the same time open your hand to grab the forearm or bicept of your opponent. As you are moving your hand with his arm, you want to bring it outside on the left hand side of your body and just basically make a big "C" with your and your opponents arm.
Step 3.) While you are bringing the arm around the outside of your body, slide your left hand that is holding the bicep/forearm down to the wrist. By the time you are finished making the "C" shape, your opponents arm should be close to your waist, with you having a firm grasp on his wrist, and his arm should be twisted upside down and depending on the size difference, his elbow is bent the opposite direction similar to a kimura.
Step 4.) Don't let go. After your arms are down near your waist, you are going to finish the motion by bringing your opponents arm back up. So the "C" shape has now become just a big circle. BUT, when the arms are down near your waist, this is when you want to begin to spin around, with your pivot foot being the same side foot that you have a hold of the arm with (left). It also helps to keep your knees bent while doing this for better leverage.
Step 5.) You should now be on your way back around the "O". By the time your opponents arm is back at eye level, it should be completely upside down, and you should be facing the opposite direction as him. With his wrist still firmly grasped with your left hand, you're gonna place his arm on your left shoulder. Make sure his elbow is on your shoulder. Your knees should still be bent a little, and you can now grab his wrist with your other hand, and use both hand to slowly pull down his arm towards the floor. When you want them to tap, slowly raise up since your knees should still be bent, you are sort of squating. The higher you raise, and the more you pull down on his arm, the worse it's going to hurt for your opponent.
This may sound complicated but it's actually quite simple and extremely effective. I can do it all in one motion and it only takes like a second and a half from start to finish. I don't understand why people in MMA don't use this armbar, it was always successful for me, and I've done it to hundreds of people of all sizes. I'll try to find a video later and maybe that could help explain it better than what I did. Once you master it, it will become your favorite technique. It's not for everyone though. I have EXTREMELY fast reflexes, (thanks to my training in Akido and TKD) so for me it's not too hard to catch a punch that is halfway to my face. Like I said, if your opponent is relentless in his attack, then obviously this move won't work, because as soon as you grab the arm, you're gonna get rocked with a punch from his other hand. What makes this move a lot easier, is punching to the body and face immediately after you grab the arm. This way he gets loosened up and distracted, and will have a much harder time fighting it. Another thing to remember, is if your opponent is expecting you to do this move for whatever reason, it is pretty easy to defend if your opponent is stronger than you. That's why I said it's best when done fast, and it's best if you loosen your opponent up with a quick jab to the face or body immediately after you catch his punch.