When you cross harmonics you get a “bump,” an audible break in the sound (think switching from normal voice to falsetto).
Generally, they all have an F valve and some other valve that will give them that low B.
One common system is one valve in F, the other in D (or the combination of the first and second will give D). Jurgen Faisst that shows approximate positions and harmonics for the B flat trombone, the trombone with the F valve depressed and the trombone with a D valve engaged (notice that with the D valve the trombonist can only get a major third): Possible Glissandi (The Exhaustive List) Using the info above we can construct layer upon layer of glissandi.
With the open B flat trombone these glisses can be as wide as a tritone.
The first conceivable gliss would be from pedal E to pedal B flat but because of its extreme low register it’s not playable by all tenor trombonists: Any glissando higher than that is playable either with the F attachment or on the open B flat trombone.
But 99.9999% of trombones today are pitched in B flat bringing us to the question of “what makes a trombone glissando possible?