Next to the line where your enthusiasm is listed, is a drop down menu called "effort"
there are three choices for effort: crunch time, normal, and take it easy.
Crunch time will make your players perform better offensively, but it will drastically reduce your enthusiasm. After a crunch time game, your enthusiasm will be half of what it was.
Normal effort will have no bonus or penalty to player performance, but also wont change your enthusiasm. One thing to note is that scrimmage games are always played at normal effort.
Take it easy will have your players play a bit worse offensively, but you will build up enthusiasm.
As you build enthusiasm, your players will perform better defensively and in rebounding, regardless of what effort setting you choose. One rule of thumb is that a team with ~8 enthusiasm and using a TIE will perform about the same as a team with 5 enthusiasm using normal effort
So, building enthusiasm against bad teams, or bots, is an important thing to do over the season. The high enthusiasm lets you play better, and then it lets you use a higher effort in important games later on.