In this video I show my primary defense/reversal in the Turtle Position. I have seen variations of this move showed many times but this differs from the way I see it most of the time as it allows you to trap the arm very securely with little to no strength and it also allows you to get your hips under the body of your partner/opponent.
The first step is to control the arm. With a lot of the different setups that I've seen the trapping of the arm forecasts the move that you're going to do and they either pull their arm out or hop to the other side. By trapping the arm the way that I do in this demonstration you will almost never have to worry about someone pulling their arm out. If they do use enough strength to clear out their arm, often, they will create so much space that you can simply sit up into a guard position.
After you've trapped the arm you need to straighten your inside leg. For one split second both legs will be straightened. Once you clear out your top leg you should replace your bottom leg in that space. When you do that your hips will connect to their hips and you will essentially have the ability to slide under your partner to make them teeter. When you do the reversal this way it will make it MUCH easier to take them over.
In the event that there isnt enough space to get your knee under the body (because they sprawl) then you should walk your pelvis as close to hips as you can. When you do this, if you are gonig against someone of similar weight, you will be able to institute some movement that may lead to a reversal.
In the event you can not reverse them because they sprawled or they are too heavy, simply force yourself back to your knees and sit to guard. The act of going to your knees will ensure there is some space for your bottom knee to sit through (especially considering when you go to their knees they'll try to rip their arm back and create even more space).
Lastly, when you do get your reversal and they fall over you should control both knees. Do not try and control the knees before you connect the hips because doing so will loosen the grip of their arm in your armpit and allow them to escape and crush you. Once you control both knees make sure to finish in a good position by shrimping your butt back while maintaining that knee control. This will angle your chest towards the floor and give yourself some space to go to a good side control position (as opposed to just trying to turn directly into side control).
In the event things go very poorly, maintain those double pant grips and you will have an opportunity to bail to a belly down position.