The X-pass for BJJ has quickly evolved into a go-go guard pass for higher level practitioners of the art. As such, it is necessary to take a step back when considering how to defend the pass, taking a look at what the person's objectives really are when attempting this technique. Here are a few preemptive and not-so-preemptive ways to stop this guard pass from happening.
RDLR and the Basics
This technique is generally a response to when you're using reverse De La Riva guard, which, in turn, can operate as a knee cut guard pass defense. Start with your partner low to the ground in combat base, and as they realize they can't execute a knee cut pass (because of your RDLR hook), they'll eventually need to stand up. Generally speaking, whenever using reverse De La Riva guard, you want to grab their foot so they can't simply kick their leg free. However, an alternate approach is to use a lower lapel grip when initially preventing the knee cut, and simply keeping this grip as your partner stands up, maximizing your ability to control their posture. While keeping the lapel grip, simply straighten your own hooking leg as your partner tries to kick free. The lapel grip will keep them from being able to get their leg free, and you should be able to reset your position in RDLR.
To Single Leg X
Building on the previous technique, as your partner attempts to kick their leg free via a classic X-pass, continue to extend your leg to follow them, while still holding onto the collar grip. Use this kicking momentum to allow your partner to pull you underneath them, while straightening your left leg (not the RDLR hooking one) under their hips, ultimately snaking it around for a single leg X-guard position. Use your left thigh to pull their right foot forward, ensuring they end up stepping right next to your hip.
Buddy Foot vs Yoga Foot
In a titanic struggle of memes and tropes, when the "yogafoot" steps up to the plate, it is ultimately the "buddy foot" that must come to the rescue. As your partner moves into a more sophisticated, calculated X-pass, their objective is a simple foot swivel, not a large, sweeping movement of kicking the air behind them. This means taking advantage of more subtle opportunities. As soon as your partner disconnects their leg from yours, you need to engage your other leg (the "buddy foot") in order to utilize a basic guard maintenance concept. Next, you have to use this post to help bring your other leg back to the center, where your partner was initially trying to pull it anyway.
Very Late Stage
Sometimes your partner is more than a step ahead with this pass, and they've prevented your ability to step over with your "buddy foot" by closing the distance with their upper body. It is absolutely imperative that you utilize a strong frame with your left arm here, directly posting on their left shoulder. This will keep them from finishing closing the gap, and should allow you to angle out much further than they initially intended. Once there, you can then step over with the "buddy foot", utilizing the same strategy as earlier to bring you back to center.
Is X-Pass the Best Pass?
Like most fundamental concepts, guard maintenance is deceptively easy. You have a certain set of tools you can use, like posts and frames or the "buddy foot" concept, and you also have the ability to deconstruct what your partner is up to, taking a good look at the guard pass itself, and realizing what vulnerabilities are inherent in the technique. On the other hand, this ultimately is destined to become an arms race of sorts, as your partner then adjusts their game in order to respond to your updates. And so it goes: jiu jitsu constantly evolves, and we can all help it do just that.