As of Oct. 1, 2016, the ITTF changed the coaching rule. Before that time, coaching was allowed only between games and timeouts. Now the rule is, "Players may receive advice at any time except during rallies provided play is not thereby delayed."
What does this mean for players and coaches? In theory, it means coaches can tell players what to do every point, either by calling it out (perhaps in a language the opposing player or coaches does not know) or signaling. It means a player can look back before each serve and the coach can signal what serve to use. It means that when a coach sees the player is doing something tactically wrong, he can tell the player immediately, rather than waiting until the end of the game or calling a one-time timeout.
In practice, it's not that simple. In most cases, calling out advice is risky as the opponent (or coach or friend of the player nearby) may know the language. In many cases, the coach might only know one language, and is so handicapped. So this type of coaching will mostly take place when a ball happens to come by the barrier where the coach is, and the coach can then whisper something to the player. This can, of course, be abused – a player may take a sudden walk around the court, getting close enough to the coach to hear his whispered advice, and will likely get away with it. (But woe be the player who is too transparent, and, say, kicks to ball toward his coach so he can go pick it up!)
In reality, except for the serve, table tennis is a game of reacting to an opponent, and trying to over-coach often turns a player from reacting to the opponent to over-anticipating (and thereby not reacting properly to the opponent). So coaches and players should be very careful about coaching during a game on most areas.
However, the serve is different. I expect that more and more coaches and players will set up signals whereby the coach can signal in serves. Some coaches will want to do so every serve. I think that's a mistake, as it turns the player into essentially a mindless zombie. Instead, coaches will likely develop signals to emphasize what serves the player should favor. For example, if a player keeps serving deep, and the coach wants him to serve short, he doesn't need to signal every serve; he simply needs a signal that says "serve short more." Or one for "serve long more." Or "more backspin serves." And so on.
That doesn't mean the coach won't want to signal (or whisper) other advise, but only sparingly. If a player is, say, playing too much to the backhand, the coach doesn't need to signal each point for him to play more to the forehand or middle; he needs a general signal for this to remind the player.
The problem with a player receiving constant coaching during a game is that he will stop thinking for himself, and so always have to rely on the coach. You can't learn to think tactically unless you are thinking tactically, and so a player who relies too much on the coach will grow up to be weak tactically. On the other hand, a player who learns to think for himself, but gets periodic signals – "corrections" – from a coach, can learn to think for himself, especially as he learns to self-correct and learn why the coach wants him to make certain tactical changes.
One practical concern is which side of the table the player is on. If the player is on his side, the coach can often whisper advice to him. When he's on the far side, he can signal to him when (with the opposing player's back to him).
So what should a player with a match coach do? Before the match, work out signals with the coach. Keep them simple; if you spend your time trying to remember what each signal means, it means you won't be thinking about what you should be thinking about, i.e. tactical thinking. Perhaps a signal about short or long serves (including fast ones, such as down the line); type of serve (forehand pendulum, backhand serve, etc.); service spin (backspin, topspin, sidespin, no-spin); placement (forehand, backhand, middle); and for more (or less) aggressive. These are some of the common things that coaches might want to say to a player in a match. But overuse of even service signals will probably be counter-productive – instead, the player should be making these decisions, with the coach giving occasional input when he thinks a major change is needed.