How to Defend Guard Pass (Double Under) - Early Stages BJJ Tutorial
This tutorial describes the early stages of defending the double under stack pass. We're going to begin with very heavy legs and hips and a straight back (described in detail below), then establish a solid position from which we can attack with several options (just two very easy ones are shown here, but there are more). Shutting down this fundamental pass early on can save your lower back from a lot of strain, and make life generally easier for you, and taking advantage of the moment when your partner tries to pass your guard gives you an incredibly good opportunity to move on the offensive quickly.
Making Your Legs Heavy!
As soon as your partner dives both arms under in order to start passing your guard, this is the time to get very, very heavy with your hips and legs. There are a few tricks that can help you accomplish this, but simply reacting with the intention to become heavy and keep your partner from accomplishing their mission (to pass your guard) goes a long way. Once your partner dives both arms under, it is imperative that they aren't able to clasp their hands together. If this happens, use the steps outlined in the later stage double under defense tutorial. Open your legs wide, and make sure that your legs are extended (a natural tendency is to curl your legs, but avoid this if you can, as straigtening them makes them effectively heavier). Once your legs are straight and wide, making it all but impossible for your partner to clasp their hands together, make sure your toes are "live" (curled upward) - this will add a last extra detail to the "heavy legs" phenomenon.
If your partner is still driving into you, it's important to make your back as flat as possible. If the surface area of your back is small, it's going to be easy to ball you up. We don't want that. Augment this by putting your head on the mat, making your back, neck, and head all in the way of your opponent's driving motion.
The Swivel Triangle
This technique is one I've used successfully in competition at relatively high levels, and it's one you can start using almost immediately, once you have a solid handle on the above basics (the "heavy hips" routine). Start by falling to your right elbow, making sure to make enough room so that you can swivel your left leg in front of your partner's face (if you're very flexible, you might be able to accomplish this with little or no leaning, but if you're normal to less flexible, make sure to fall all the way to your right elbow).
Once you're leaning on your elbow and your left foot is in front of your partner's face, you can kick your left leg through and underneath your partner's armpit. At the same time, drag their arm across, using the handy (no pun intended) sleeve control. This will help ensure that the triangle is already tight. From here, it's just a matter of pivoting so that you can look into your partner's left ear, making the triangle perpindicular. For some bread and butter basics on finishing the triangle, check out this tutorial on escaping the triangle choke (focus on the first video).
Shoulder Lock From Double Unders
Starting with the same initial position (heavy hips!), and sliding or walking backward to establish the feet on hips/sitting up position from before, let's take a look at another basic attack. Use your hip movement (as described) to shoot your left leg all the way across your partner's hips to the other side, so that your left foot is hooking their left hip on the far side. Keeping a firm grip on their sleeve hand, just extend your left leg while basing on the ground with your right leg. This bends their right elbow inward, much like an Americana submission, forcing the tap due to a shoulder lock (note: go slowly with this one at first, as you might be surprised at how effective it is!).
Other options for attacking from a strong double under position can be found here, in case these aren't working for you. There really are a lot of options to attack any time your partner opens up and goes for a pass on you. If you're a little bit later in the sequence, start with these options instead. As always, play loose and open so that you get the chance to try these out on your partners during live rolling, and let me know how they work for you!