It's pretty basic stuff and if you don't understand it you're highly unlikely to be successful at testing anything no matter what other methodology you use.
If you have a finite amount of time to test something in the practical environment in which it is going to operate then you need to use that time wisely. That means turning up to those test sessions with an idea that you have already thought through, developed and tested, short of putting it on track, and where you know that's the direction you will go in. You don't have time to change your mind later.
For example, Renault turned up with their exhausts in a forward position and they've stuck with it and given themselves the maximum amount of time to tinker. Red Bull added a new exhaust positioning on their car and they've stuck with it. You don't have time to fully develop the direction you're going in if you're radically changing the a component every day or so. It also means that you still have no idea what direction you're really going in. This is also from a team that skipped the first test because they wanted more development time.
If you had unlimited testing like we did a few years ago then you could do live development on track, or certainly the larger teams could. We don't have that now.
Seriously, it's basic stuff.