I am not experienced with full speed sparring in FMA yet, and in fact I haven't trained FMA very much, so I speak less from specific experience and more from cumulative intuition/understanding:
I likely don't know what I'd do. It depends on range and timing and reflexes, and whether or not I see/feel the opportunity to disarm, defang, or attack some central target.
Having a Wing Chun background, I am cautious of chasing hands and prefer to threaten the opponent's core in some way, especially if the range is such that both you and your opponent are in range to threaten eachother's head or torso. Chasing the hands or the weapon is a good way to get cut/hit/stabbed. In my short time practicing HEMA, this is something I made good use of against less experienced practitioners, who tend to cut to the sword rather than the target. If you're swinging at my weapon, you're not threatening me in any way, so I just move my weapon out of the way, avoiding/ignoring yours and attacking you at the same time. It's pretty easy to tell when someone is chasing the hand or weapon and do this.
In random weapons sparring, mostly in the context of knife defense, but also a little bit of light stick sparring under pressure (the latter I have done far less of), the disarms that I pull off have mostly been those which just "fell into place." Some of them I went for and succeeded, some of them I just "found", and some of them were accidental. My very first disarm in sparring ever, in fact, was accidental: in one of my Wing Chun classes someone asked about knife defense, and we paired up and practiced knife versus empty hand, where both attacker and defender were free to do anything. Welp, one of my very first instinctive reactions to a stab that came in was inside pak-sao (slapping the inside of the wrist). The shock bent my attacker's wrist and caused the knife to go flying out of his hand. Successful and completely accidental disarm. I didn't even intend to do an inside pak-sao either -- I usually avoid that as it leaves you exposed to your opponent's rear hand. But it was in position to deal with the threat, and it did!
Not just with disarms, but with all techniques in general: I find that the more I "go" for a given technique, the less likely it is to succeed. The techniques that work are those that come out naturally when the situation presents itself, and my approach is to just try to train in such a way that I learn to put two and two together and recognize opportunities that are presented to me. Of course, training more complex techniques such as disarms with increased resistance and/or speed is important too. This sort of practice has to be done carefully though because if the opponent can resist freely and you're limited to just one technique, of course your opponent knows what you're going to do and can easily stop you from doing it. That's what I dislike about most knife defense "debunking" videos you tend to see: of course some technique that you've hardly trained before isn't going to work when your opponent is resisting fully, you're limited to a single technique, and he even knows what you're going to do. You need to be able to respond freely as well, and that means having a tool box of techniques and well trained reflexes, and flowing into whatever is suitable when you do counter resistance or something doesn't work.
