Want to have your mind blown??
Try doing this exercise where you try to trace the outline of a star by looking in a mirror. If you haven't tried it, it isn't easy!
Right is left. Left is right. Up is down. Down is up.
I imagine if you practiced it for three hours a day, though, you'd get good at it. Then you can impress your friends with your new-found skill!!
(uh, maybe not...)
At first, your drawing might look like this:
Of course, by now you've figured out that I'm actually talking about slide technique. Think of each of those clean, sharp angles as a slide position.
Without becoming tense, you want your slide movement to perfectly "outline the star" of the position sequence you're attempting. For beginning trombonists, it isn't easy. I imagine if you practiced it for three hours a day you'd get good at it.
Instead of a star, imagine this crude drawing of a G-major scale (using the B in the lowered 2nd position with the valve)...
You want crisp, accurate slide movement, not that curvy approximation stuff.