Their terminology is odd. The thing you move while playing is generally called the hand slide. There's nearly always a separate tuning slide located in the crook of the bell section.
(Some relatively rare instruments like the Shires Alto do "tuning in slide" with a mechanism for fine adjustment in the hand slide).
If you're also moving the tuning slide in the middle of a piece you're probably a bass trombonist doing the now-impossible glissando (see https://www.youtube.com/watch?v=fWJPeA_1g48) in the Bartok concerto for orchestra.
I was thinking the same thing. The tuning slide is not what you use while playing, it's the separate slide on the bell side of the trombone for fine adjustment to ensure you're in tune with the rest of the band.
Do you mainly control pitch with the tuning slide or the valves on those instruments? I think you mainly control pitch with the valves and only supplement with a tuning slide for certain notes, depending on the instrument, and therefore the statement is accurate.
Mainly the valves. The tuning slides help with a number of things, including the fact that the harmonics (notes above the first ocatave) are not precisely in tune with the fundamentals. A trumpet typically has a trigger lever or a loop for your finger on one or two of the tuning slides.
You use it as needed. If you're playing a really fast passage, you'll likely skip it, but shorter notes are harder to place the precise pitch anyway.
If you really want to see tuning slides in action, find a video of a good tuba soloist.
Interesting. A student horn I guess? I've never owned one that didn't have at least a third valve slide ring. I think they were widespread around the time that the trumpet really took over from the cornet, around 1900-1920. My pre-War Olds has moving slides.
The interrupting parenthetical was so disruptive to the sentence that I thought it said, essentially, "The trombone is the only brass instrument. Parenthetical. The trombone is played by moving the slide."
(Some relatively rare instruments like the Shires Alto do "tuning in slide" with a mechanism for fine adjustment in the hand slide).
If you're also moving the tuning slide in the middle of a piece you're probably a bass trombonist doing the now-impossible glissando (see https://www.youtube.com/watch?v=fWJPeA_1g48) in the Bartok concerto for orchestra.