The heel hook is a powerful tool in both grappling and MMA. While heel hooks are more common in grappling, they may be quite useful in MMA if used correctly and keep you away from punches. The UFC has just published an intriguing video showing all heel hook finishes from 1993 until 2020. It's a great way to see how often this submission is used in real fights.
In general, yes, you can heel hook in the UFC. However, it is not easy because your opponent will do everything he can to avoid it and break away from it. Also, be careful with how often you use it because if someone knows they can beat you by doing so, that will give them motivation to fight against it.
Here are some examples of heel hooks in action:
Khabib Nurmagomedov vs. Alistair Overeem (2016): This is one of the most famous heel hook battles. Khabib uses it on Alistair multiple times during their fight at UFC 223. He even uses it as a finish when they go to overtime!
Tony Ferguson vs. Michael Chiesa (2018): During this fight, we see Tony use the heel hook twice to win him the match. First, he does it at 3:40 into the first round. Then again at 7:20 into the second round.
The left hook, like the other foundational punches, generates power by pushing into the ground from a strong boxing stance: keep your feet shoulder width apart, with one foot (typically your left) slightly in front of the other. As with all powerful punches, aim for the head or body cavity.
In boxing, the left hook is used as an offensive punch to land on the opponent's cheek or to end a round. The punch can also be used as a counter-attack when faced with a straight right hand from your opponent. To execute the left hook, quickly step out with your left leg and drive it into your opponent's chest/stomach. Then snap your arm back up toward their face, turning your palm upward in order to look like a hook. It's important not to pull the elbow back too far, or else you won't get the full effect of the blow.
As with most punching techniques, perfecting your left hook requires practice. First, stand with your feet about hip width apart and hold a light sparring bag with both hands. Swing the bag back and forth in a smooth motion so that you mimic the movement of a boxer throwing a punch. When you feel comfortable doing this, switch to using only your left hand to throw the bag away from you. Practice until this movement produces the same result as actually hitting someone in the face with a left hook.
In UFC 4, you may start a grapple in one of two ways. Take the opponent to the ground (L2 + Square on PS4, LT + X on Xbox One) or start the clinch (R1 + Square/Triangle on PS4, RB + X/Y on Xbox One). You can begin grappling from the mat or within a clinch. When starting a grapple from within a clinch, choose an appropriate time to strike -- either after a brief pause or after another successful action.
On the ground, you have three options for how long you can hold your opponent down: normal, extended, or invincible. Normal holds last for four seconds, while invincible holds last for eight. If you want to extend a hold, simply tap L3 + R3 at the right moment. This will cause your opponent to roll over and be vulnerable to additional attacks.
After a few successful grapples, you will earn the option to perform leg sweeps. To execute a sweep, stand next to your opponent with both of you facing the same direction as soon as they go to stand up. Then, press the Circle button to pull their legs out from under them and send them crashing to the mat.
You can also attempt to submit your opponent by pressing the X button when they are on the ground. However, if they block the submission attempt, they get a free hit off of whatever attack you used to try and submit them with. They will not gain access to their weapon until after this happens.