This is really hard for me to define. I guess the most concise way to put it is like this:
If I feel a guy going very, very hard, but clearly lacks basic skills and knowledge, I play very defensive. One - to guard myself against injuries due to them spazzing, and two - to protect them from similar injuries. Since 90% of what I practice is from standing, this is even more important. My only lasting injuries are from takedowns and throws. Also, I want new guys to want to come back. I want them to develop as players over time. If I have too much of an ego to take a few bad positions from a new guy, or god forbid a sub (oh no, how will I sleep at night?), then I am letting my emotional state impede another's development. Mutual benefit.
Conversely, if it is a skilled practitioner, and they are going hard, I try to exceed their intensity, because I know the training is under control, and intensity is a necessary component for creating progress at an advanced level.
I used to coast through all my training, until I realized that I wasn't getting better as quickly as I should. But I had to develop a certain level of skill before I could add 100% intensity, or that intensity was all for naught.