I was first exposed to X-guard around 2002, and although it was being tinkered with before then, it wasn't until 2003 when Marcelo Garcia really brought the position to the forefront of the collective sport BJJ brain. Marcelo won all of his matches to win gold that year, and used X-guard sweeps to seal the deal in most of the matches. As a consequence, the familiar sweeps from X-guard are very well understood, and often preemptively defended. We're going to throw a monkey wrench into that.
The kneebar is the most conventional X-guard move we're doing here, and it's mainly here for reference. The classical X-guard allows you to get deep underneath your partner. While here, one strategy to sweep is to simply take their "table legs" away and to look for a sweep. Start by gripping the nearest sleeve. Next, retract your bottom leg and step on their triceps. This will essentially isolate their near leg. Now, simply push the triceps and pull the sleeve toward you. You should be able to direct their fall/roll right into a kneebar submission.
While most advanced grapplers are familiar with the arsenal of sweeps available to them, few are playing with modified X guard. Start with a single-leg X-guard (more on that below), and as your partner tries to turn their knee away, use your foot to hook their leg, bringing it back to the direction you want it facing. Be sure to step on their ribs here to lift them off the ground just a hair, off balancing them backward. Turn your knee outward to execute a sweep, and as they're falling down, make a tight figure-four straight ankle grip. Finish by stepping over to their hip.
Single Leg X-Guard
Here's a classic sweep and finish, but with a twist. Start with double butterfly hooks behind your standing partner, and scoot underneath them, shooting one leg through the middle, while maintaining your hook on the other leg. Next up, step on their hip, keeping "live toes" pointed outward. Move the inside foot and shin behind their knee, and use your shin to direct their knee outward, forcing them to sit down. While they're sitting, you can hit a classic straight ankle lock, but here, I pass the foot to the other side and finish a nice cross-body ankle lock.
Deep Half as X-Guard
Technically, this series is from deep half guard, but this works just as well from X-guard when your partner drops the knee closest to your head to the ground (ostensibly to avoid the "stand in base" sweep). The general idea is to make proper X-guard hooks, and then simply to shoot your legs through, following where your feet already are. This will enable you to land in a 50/50 position, but you're going to have a significant advantage over your partner since their heel will be exposed during the fall, and because you're initiating things. You really need to use your feet as hands during this move, making sure to pull their foot inward toward you, forcing them into the position you want. Pass their foot to the other side of your body, and heel hook to your heart's content (and at your instructor's discretion, of course!).
Deep half or X-guard?
While I heartily recommend really taking the time to learn how each position works before simply diving in and attacking leglocks, X-guard is a natural fit for lower body submissions. Nevertheless, the more you understand the techniques involved with the aforementioned basic X-guard sweeps, the more likely you'll be to have success with the "fancy" leg attacks shown here. As always, check with your instructor (and training partners) before slapping any surprise submissions on (particularly heel hooks), but by all means, please try these out when you get the chance, and let me know how it has gone!