What does Herald's affinity chart mean by "Use a Special"?

I have gotten a rare blade named Herald. Both her unique field skill, and her key affinity rewards, require her to "Use a Special" a certain amount of times for each unlock. I've gotten into battles where I use blade combos, driver arts, driver combos, and every level of blade art, yet I've only gotten 1 out of 11 progress completed, and I don't know what triggered it. So what exactly does special refer to in this case?

I'm fairly certain you need to use special Blade arts (the one you activate by hitting A). I distinctly remember swapping to herald, hitting for a while and then spamming the special ability.