One of the axioms of coaching is that you can't fix technique in the middle of a match. Whether coaching someone in a match, or trying to make adjustments yourself in a match, technique is something that needs to be honed in practice so it becomes automatic (i.e. subconscious) in an actual game. The reason for this is that the game is mostly played subconsciously. You don't consciously do technique very well in a game situation, or even choose the proper racket angle - your subconscious does all this after you've honed it in practice. There are just too many things to remember to do for one to do so consciously; the minute you take conscious control all that training is out the window. To fix something in mid-match means to consciously take control, and that rarely goes well.
This is why the conscious mind's task during a match is mostly tactical - choosing what serves to use, how to receive, how to rally, etc. (And even there, much of it becomes subconscious - for example, when your conscious mind decides to attack deep serves, for this to work the subconscious has to get the message so it does so automatically.) The conscious mind should also focus on sports psychology - but that's basically techniques to get the conscious mind out of the way so the subconscious can take control.
There are a few times when you can make basic fixes to stroking problems, though not necessarily technique problems. (I will write this as if I was coaching a player in a match, but you can do this yourself, i.e. coaching yourself in the same way.)
Suppose your player lifts way too much when looping or smashing, and keeps going off the end. (There could be a dozen technical reasons for this; he might be standing up too straight, or using too much arm, or not transferring his weight forward, etc.) Trying to fix his technique directly probably won't work - that's something you'll have to do after the match, when you can zero in on the specific problem and drill the player to do it properly. So what can you do? Instead of trying to fix the technique, understand that the poor technique is putting the ball off the end when it aims for the table. So change the target; tell the player to aim for the top of the net. If the player makes a conscious decision to aim for this, and then lets the subconscious do this, it'll probably get the message and aim for the top of the net - and will likely do just what it was doing before, i.e. aiming too high, and so it'll instead go over the net and hit the table.
Suppose your player has, say, a wristy forehand that causes him to lose control. Telling him to change his technique isn't going to help in that match; it'll just put him in the position of trying to learn a brand new technique, i.e. a wristless forehand he hasn't used before. His subconscious doesn't know what to do, and so you essentially turn the player into a beginner, at least with this one aspect. You could, of course, think of the match as just practice and have the player focus on the technique. Then it's not really a "match" so much as a practice session. But it's a highly inefficient practice session as the player will only haphazardly get to use the shot in question, and when it comes up, guess what? He'll reflexively do what his subconscious is used to doing, i.e. the poor technique. That's why to fix technique you have to practice it in a drill situation, where you do it over and Over and OVER until it is so honed that the subconscious can do it in a match. If it can't, then it'll fall back on old habits during the match, and re-enforce the bad habit.
Suppose your player is standing up too straight, which leads to all sorts of technique problems, such as lifting too much when looping. If he's used to standing up straight all the time, there's not a lot you can do in the middle of a match. On the other hand, if the player sometimes stays low but other times (perhaps in the pressure of a match) stands up too straight, reminding him to stay low is a simple message that the subconscious can get since it's used to staying low, and it solves the technique problems.
Sometimes it's not a technique problem so much as adjusting to something the subconscious hasn't seen before, and so isn't able to react properly. For example, suppose my player keeps blocking an opponent's slow, spinny loops off the end. The player hasn't seen such spinny loops before (at least not enough for the subconscious to learn what to do automatically), and so nearly every block goes off. I could tell him to close his racket angle - but that probably wouldn't work. In a game situation the subconscious automatically chooses the racket angle, and trying to adjust this against these super-spinny loops in the middle of a match is slow and difficult. If you tell your player to close the racket more, and he tries to do so consciously, he will likely lose what little control he had as his conscious mind can't reflexively do the proper angle in the split second it has to do so, and will tend to fall back on the reflexive angle that wasn't working before. So consciously telling oneself to close the racket more isn't really that helpful - the subconscious can see that it's blocking off the end, and is already trying to make adjustments (often too slowly) if you let it. (The more you play the easier these adjustments become.)
However, if you tell the player to block more aggressively, then the spin won't take on the paddle as much, and the player won't be as susceptible to heavy topspin as before. It's a simple message to the subconscious - "block harder!" - and as long as it's not overdone ("harder" doesn't mean "smash"), it will often solve the problem.
Here's my recommendation: focus on developing proper technique in practice, not in matches. Work with a coach or top player if possible, or watch videos. Get the technique down before using it in serious matches. It's okay if you don't yet have the timing for the techniques as long as you are doing it mostly properly. Matches are a great way to learn, but they are not the place to develop technique.