Go full throttle without pressing the kickdown switch, and the transmission usually won't downshift into the lowest possible gear. The computer deems that it's not the "most urgent" request for WOT. It will typically bring the engine to around 4000rpm before accelerating, or might not even downshift at all, depending on the scenario.
Now, go full throttle while pressing the kickdown switch, and the transmission deems this as an urgent request, meaning it will put you into the lowest possible gear allowable at your current speed. I have seen the transmission downshift so aggressively, it began pulling from around 5700rpm!
Picture this scenario. You're cruising at around 80km/h at 1500rpm in 6th gear. You go WOT to pass, without pressing the K/D switch. the transmission will likely downshift to 3rd gear. Now, in that same scenario, with the K/D switch pressed, it will likely downshift to second gear instead.
For example. A real scenario that happened to me once: I went WOT to pass. Oncoming traffic came around the bend. I had little space left. I clicked the K/D switch and it automatically dropped one more gear, giving me just enough acceleration to merge in time. Using it is very hard on the transmission and hardly ever necessary. Use sparingly.
The only other thing i've noticed is where the transmission actually shifts. Go WOT in say, second gear. Urgent, but not the most urgent request. The transmission is programmed to shift as high as reasonably possible in a scenario that it assumes is, urgent, but not K/D Switch urgent. It seems to shift somewhere between 5500-6000rpm on average, depending on conditions.
Now, try WOT with the K/D, and it will go all the way up to redline, without any exceptions that i've seen, though I hardly experiment with anything near that RPM.
I have not noticed any difference in behavior between holding the switch, vs. simply clicking it. Appears it's designed to be clicked briefly.
Messing around with this switch while hooked up to Mazda edit, I noticed that you get more throttle opening, as the pedal is obviously closer to the floor, but during actual WOT testing, the ECU won't actually allow the additional throttle position. Let's say the ECU determined that 86% throttle position at WOT is the most it can give you without impeding on drivetrain safety, there's not a thing you can do to get that figure any higher. Not that you would want to! I've never seen the throttle open up any higher than 86% while tuning.
So, in short:
WOT with the K/D switch is essentially asking your drivetrain for everything it's got.
pressing the switch will probably speed up the upshifts somewhat, maybe alter torque converter lockup slightly, but nothing noticeable to me in that regard. The downshift speed, aggressiveness, and the redline shift are the main two things i've noticed.