Andrew Smith is a 3rd-degree BJJ black belt based out of Richmond, VA (Revolution BJJ). He runs BJJ Path, a video tutorial website.
Overhook to Triangle
What follows is a basic tutorial on two different options, both incredibly effective, and complimentary. Each action is completely dependent on your opponent's reaction: if option A doesn't work, option B is quick to follow, and vice versa. The triangle choke from closed guard is a time-honored classic that is constantly being updated with subtle details and positional tweaks. Here are two of my own personal favorite setups.
Basic "Jump Rope" Triangle
Getting to the overhook can be accomplished in a variety of ways, but one very reliable gi setup involves grabbing both lapels to control your opponent's posture, and then keeping the left lapel with your right hand (to keep your opponent from posturing up) while swimming around from the back of your opponent's right elbow with your left hand (Eddie Bravo calls a similar move "the zombie"). Alternatively, you can grab the back of your partner's head (as I do in the video) in order to facilitate the posture pwnage. Once your overhooking hand has swum through, grab your own lapel to reinforce the grip (this can be much more difficult to yank free than their lapel!), and then drive your right elbow into the crook of your partner's armpit, seeking to create enough space for you to drive your shin through. Slide your right hand down behind your partner's left triceps, taking care to keep your partner's arm in place. Next, slide down to their wrist, and apply pressure outward with your shin. This will often cause your partner to retract their arm, and then you can bring your knee in to your chest (think: jumping rope).
The Devil's in the Details
In this variation, I opt to grab Daniel's collar instead of mine, but the gist is the same here: your partner's posture is controlled effectively with one arm, and then you can have your other arm completely free to attack for the "inside space" with your shin. Once again, the "jump rope" effect is best achieved after getting a reaction from your partner, as your shin slices right into their biceps. For the finish, if your legs are long and relatively thin like mine (and your partner isn't the size of the Jolly Green Giant), you can actually leave the overhooked arm on the outside of your hip, bypassing the classic "hips up, arm over" phase of the typical triangle finish. All you really need is the right angle to finish, which is often best achieved by hooking your partner's arm in an underhooking style.
Stuffing Triangle Option
Here's another very simple (but effective!) option. You're in the same position as before, trying to hit the "jump rope" triangle option, but your partner counters this by not reacting to your "shin slice" into their biceps, and by squeezing their left arm in tight (essentially the opposite direction in which you'd like them to go if you're going with the previous attempt). Once your partner squeezes inward, this is your cue to switch your direction of attack: simply push the wrist inward to your partner's chest. Now, get your leg out of the way. Just kick your right leg out at a 45-degree angle, and then make sure you hop over your partner's elbow (see more on this concept at overhook triangle setups).
I can't tell you how successful these setups have been for me over the years, and they've been even more successful for some of my students. The great thing is that this sequence is loosely based on a self defense punch-block scenario that many newer students learn. The funny thing about this is that techniques like this are often dismissed for sport jiu jitsu competition, but some of those moves are among the best setups available. These triangle setups are certainly no exception to this observation. Test them out with your partners, and see if you can catch an unsuspecting partner off guard a few times. The best test is when they know it's coming and can't stop it anyway, and you can definitely pull this phenomenon off with these techniques.