Professional coaches always seem to know what to say to a player between games or in a timeout. That's why they are professional coaches. A good coach has essentially seen it all and can see the patterns that lead to tactics that will work in that match. They also understand the psychology of table tennis and know how to best get their player into the right frame of mind.
But suppose you are coaching someone between games, and you aren't a professional coach or top player who has seen it all, and aren't all that sure what to say to the player between games and during timeouts? (A good example of this would be parents who are coaching their kids.) Here's what you do - and #4 and #5 is probably most important.
- Speak slowly and calmly, even if it means saying less. If you sound like a nervous wreck, think how that's going to affect your player.
- Keep it simple. What's the best serves to use? Best receives? Best rally shots and placements? No more than 2-3 things is best.
- Be specific. Don't tell them to focus on better receives; tell them what specific receives work better.
- Find ways to raise their confidence. If they look nervous, don't tell them to relax - that doesn't work. Tell them to imagine it's just another practice match at the club. Have them stare at something in the distance for ten seconds while clearing their mind. Tell them, "You can do this." Simple statements like that work. Ask them what their game plan is - a lot of nervousness comes from not being sure what they should do. Having a general game plan, and actually stating it, often fixes that. Just as importantly, thinking tactically keeps one from thinking about winning and losing. You can’t think about two things at once.
- If you aren't sure what to say, ask questions. Ask them what their go-to serve is - that's a good way to make it clear in their mind what serves they should use. (I am a professional coach, and I often ask this of my players, even if I know the answer. If he gives a different answer, then perhaps he has confidence in that serve - and then I suggest what I had in mind as another serve to use as well.) Ask how he thinks he should receive the opponent's most common serve. Ask where he thinks the opponent is weakest. Ironically, these questions get two birds with one stone - they not only get the player thinking tactically, but they also get their mind off winning and losing - see the end of #4 above.
And there's a great benefit to coaching someone even if you aren't a professional coach - it gets you thinking tactically while seeing real-world tactical issues, and makes you a better tactical player. Good luck!