Andrew Smith is a 3rd-degree BJJ black belt based out of Richmond, VA (Revolution BJJ). He runs BJJ Path, a video tutorial website.
The monoplata is a crafty submission popularized by Marcelo Garcia in the late 2000s. The technique is a shoulder lock from mount or 3/4 mount (both options are explained here, along with the pros and cons of each). While the omoplata has been in the submission grappler's vocabulary for the better part of 20 years now, the monoplata has only recently started to become an understood and valid attack at higher level BJJ competition. Here's a quick breakdown of the basics of how to set up and finish this extremely sneaky submission, along with some preemptive troubleshooting.
3/4 Mount - Tight and Tough to Escape
The pros of setting up the monoplata from 3/4 mount include the extreme difficulty in escaping the position, and the utter sneakiness of the technique. Many folks will let their guard down when they have a foot trapped, feeling like submissions aren't really applicable until you pass their guard. This scenario starts when your partner has captured your right foot after being mounted (as shown in this tutorial), or when you haven't fully passed their guard but have ended up 3/4 of the way to mount. In this case, your partner is taking a right underhook. This is far from ideal defensive posture for your partner, but sometimes they'll end up taking the underhook anyway. Other times, you can sort of force the underhook to happen. At any rate, the first order of business is to keep your partner on their side, ensuring that the underhook stays trapped and there's no going back to proper defensive posture. Note that my knee pinches inward in the video shown, with my toes out. Next, use your right hand to push your partner's head down, and base out with your left hand to slow down (and tighten up) the following steps. Keeping your left knee pinched, pass your left foot in front of your partner's face (think: omoplata). Now grab your left shin with your right hand, closing off all remaining space, and augmenting your ability to rotate. You can finish by turning your hips to your right from this position, or you can (carefully) sit down to finish the rotation. Your partner's hips are still trapped by your foot (still between their legs), so rolling to escape is all but impossible.
Option 2: From Mount
The pros of the setup from mount are that you don't need to be as flexible to set the monoplata up, and if you mess up, you're probably still going to be in the mount position. The con: your partner may be able to roll out to escape, or rotate their arm (but we'll deal with the latter). This is easiest set up when your partner bear hugs you from bottom of mount (again, not ideal defensive posture, but it does happen). Start by doing what Daniel calls a "diagonal push-up," creating an angle similar to the previous technique, focusing on trapping your partner's right arm in a perpetual underhook. Next, try reaching under your leg and around your partner's shoulder (think: Shawn Williams guard). This will help to secure the position previously described. Create a frame across your partner's face in order to release their grip. Next, open your right knee up for base, then pass your right foot across your partner's face (feel free to base with your right hand while doing this step). Now post with your left hand and reach across your shin with your right hand, beginning the rotation as you sit down (alternatively, sit first and then rotate for more control and safety). Because of the relative looseness of this position (mount as compared to 3/4 mount), your partner may be able to turn their thumb to rotate out of the shoulder lock. Fortunately, the transition to traditional armlock (juji gatame style) is very easy from here.
Remember when practicing the monoplata that the onus of safety is on you. Be sure that you take measures to slow down the submission as much as you need to in the beginning, gradually increasing intensity and speed. You'll likely find yourself catching a lot of good grapplers with this technique over time. As always, let me know how these techniques are working for you!