In this video I show a high percentage way to avoid being stacked and then take the back. This move looks too simple to work, but trust me, rep it out.
One of the most important things that you have to resist is your opponent clasping their hands around your waist. As you move through the ranks you will find it easier and easier to do. Fighting them off will be a combination of hand fighting and extending your back and legs to push yourself away.
It's in this time that you take control of the sleeves (which further combats the clasping of their hands together) and put your feet on the hips. Take the time to push with your feet on the hips. Pushing on the hips will open up some space. This will make space for your feet to go into the pocket by their hip while also elongating your opponent's arms so that they lose leverage.
Once you've made that space you should insert your foot into the space by the hip. When you do this it's very important that you straighten your leg to lock their arm in place (reference the video). This arm trap is VERY effective. If you feel, or your training partner feels, like there is a way to pull their arm back once you've straightened your leg then you're not doing it correctly.
Now that you've trapped the arm it's time to bring your other leg into the mix. I show in the video that you can either control one side like this, or both sides, but ultimately you're going to load your legs up on one side.
When you tilt your body it will make a little space for your leg to fit nicely next to your first leg. When you put this leg over the shoulder and under the leg it's time to straighten it just like the first leg.
PLEASE PLEASE PLEASE be careful when straightening the leg. Depending on how flexible your partner is this can range from tight, to uncomfortable to an actual submission (making the shoulder like it's going to blow up). So while you're figuring out what your opponent's shoulder flexibility is, make sure to take this move slow.
That top leg is going to be coming right over the top of that shoulder. Once you have the top leg in and straightened you can take out the bottom leg and start to turn towards your partner/opponent's back. The act of turning in will put a lot of pressure, and thus domination, on that arm. Be careful with this transition.
Keep in mind that you already have one hook in and your opponent is basically flattened out. This will give you the edge in securing the back. I prefer the Seat Belt Grip or the Tight Waist.
Try it out and let me know what you think.