- attack/defense techniques, where the two practitioners take turns in striking and responding
- escape techniques, with the attacker starting from a grabbing or grappling position
- kihon kumite, with both doing a longer sequence of techniques
- bunkai, an even longer sequence of techniques adapted from a kata, and their counterparts
In a class, you can't always choose who your partner in training will be. Every day it's someone else, and we also rotate a lot during each session, so at different times you will find yourself partnered up with someone either more experienced, less experienced or about as experienced as you are. What I think I've learned so far is that in each case you should really adopt a different mindset.
Training with someone more experienced
This is probably what everyone is looking for most of the time, because it simply provides you the best chance of improvement. For example, in our yesterday's training we were in odd numbers and I had to team up with the master for practicing bunkai, and he brought the exercise gradually up to a speed and power level that I didn't even imagine was possible for me to try at my stage of learning. But I was forced to push myself up to match the pace, out of my comfort zone, and it gave me a feeling of a real karate training like no other!
Anyone more experienced has been there and done that for you already, can provide suggestions and set the pace of the exercise to match your personal level and characteristics. When you are on the less experienced side, the most important thing is trust, and the bigger the experience gap, the more trust you should put in your partner's instructions and feedback.
Another good point to keep in mind is that you usually don't need to pull your punches. You finally have the opportunity to go full throttle (if the exercise itself allows so) and do it at your very best, because the more experienced partner can control the exercise with ease and nobody gets hurt.
Training with someone equally experienced
Apparently this should be the most obvious choice, in fact when training on your own with some friends it is quite normal to team up with others at the same level; this way everyone is going to benefit the same from the training. But there is a risk to watch for: that of both underestimating each others' capabilities, and settling for a pace, speed and power which are too much within the comfort zone.
Both persons getting out of their respective comfort zones at the same time may not be as easy as it sounds, because in such case both will probably be making mistakes that can decrease the effectiveness of the exercise. Furthermore, the two may be good at different things, one may be faster and the other stronger, meaning their comfort zones "shapes" are different.
My opinion is that the key for making it work is variation: make sure you stay long enough on each exercise and try to "dial" its different aspects so that both people are challenged enough, if not simultaneously at least at different times.
Training with someone less experienced
In this case I believe being earnest and responsible is paramount. In a way, when I practice an exercise with someone less experienced (not a very frequent case for me, since I'm only at the second-lowest rank!) I try to keep in mind that it is always the less experienced person at the center of the exercise: he is more important than me, and my responsibility is to provide him an appropriate level of challenge. It's essential to realize that both trivial and impossible challenges don't really make someone improve, because if too easy means no effort, on the other hand too hard means to easily get stuck doing it wrong all the times just to desperately try to keep up.
So rather than making an exercise too difficult for your partner just because you can, you should be willing to give up your own chance of benefit for this round: after all it's your duty to help the less experienced just like the more experienced do it for you when you're on the other end. But no regrets, because even if doing this exercise well within your comfort zone may not bring much direct improvement to you, teaching it will! Teaching something is always the best chance for better understanding: the need to observe your partner's difficulties and correct his mistakes raises your own awareness on how things should be done correctly and you'll have fresh ideas to try out to solve your own problems with the same exercise later on.
One more thing I have learned (from being on the less experienced side most of the times, that is) is that providing feedback is very important, but you should make sure to focus on clear, technical comments. Expressions of encouragement ("you're good!") or disappointments ("you need to do better...") are the kind of comments that cause only distracting feelings while practicing, so they should be left for after the training. But during the exercise itself, the less experienced person truly needs directions: if something is wrong, tell him what to change; if something is weak (power, speed, aim, flow of motion, spirit...), point it out so that he can focus more on that aspect; and if something is just right, let him know plain and simple, so that he is confident on being on the right track!