A scrimmage is usually a practice game in which a team plays its own second unit or another team. Scrimmages don't count towards a team's record. In a regular game that does count, the players perform at a higher level because of the stakes involved. In the sentence above the word "execution" is key to understanding the meaning. Against a team's first unit, the players' execution, that is, they're level of play, is heightened. Idiomatically, one could also say "They brought their A game."

I'm sure you've read the full text about basketball, so this sentence means that during practice, you can focus more on defensive blocking instead of scoring a basket.