Something to note about the torque on demand system is that it's still a wet clutch pack system that mainly works when there's a difference in rotational speed between the two axles. So basically, if the front axle (and therefore centre diff input shaft) is spinning faster than the rear, the clutch packs swimming in hydraulic fluid begin to engage due to the shearing force created (similar to how a torque converter in an automatic transmission works), and fully engage when they are hydraulically pressed together. This means that under normal circumstances, when the front wheels don't have the traction to cope with the demand, engine torque will be sent to the rear axle once the fronts start slipping. The ELD button engages a solenoid in the centre diff, preloading the clutch packs and forcing constant 50/50 torque split. The other aspect is the aforementioned simulated differential lock that brakes wheels that are spinning faster than the rest.
There are, however, several limiting factors in the functioning of this system, some of which are common with 4wd applications. For example, a real differential lock means that both wheels on any given axle are forced to spin at the same speed. On high grip surfaces, this poses a problem when turning because the outside wheel has more difference to cover. That means the lateral torque from wheels that should be rotating at different speeds to cover different differences throughout the turn is all pushed through the axles and diff. Eventually, this can cause something called "binding", basically the torque mangles the axles, driveshafts, diffs. How serious this issue can be is also dependant on many factors, such as how much power is being sent through the axles, how much traction there is, how beefy the components are. The same is true for the brake based ELD system in that while the Panda doesn't really have the power to overload the system on its own, the brakes have the stopping power to clamp down enough to stop any spinning wheel. If one is spinning but the other isn't, anything in between can get seriously damaged. That's why mechanical limited slip, torsen, or even locking differentials can still work at higher speeds but electronic systems need finer tuning to work. This is part of why the ELD system doesn't work above 30mph/50kph, and explains the side to side involvement of the 4x4 system (with ELD enabled, disabled it's just an open diff and stability control does a limited amount of braking if pushed too hard).
The mysterious centre diff is also limited in this regard, and for a similar reason. In vehicles with a real locking centre diff, there is a miniscule amount of gearing that takes place in that diff which enables the front axle to turn slightly faster than the rear. It's not noticeable at low speeds or on low grip surfaces but high speed high and/or high grip surfaces cause the same binding effect, just front to back rather than side to side. Most drivetrains can cope with a certain amount of negative load in the driveshaft and centre diff, so they can withstand a bit of being pulled apart, but being pushed together mashing everything together puts directional load in parts and places they can't handle, not to mention the entire chassis essentially trying to withstand being squashed together like it's being crushed between two trucks. This is why locking centre diffs on serious off roaders can only be engaged at low speeds, on low grip surfaces. There are, of course, exceptions to the rule but then we're talking about Icelandic hillclimb trucks or rallycross vehicles costing well over half a million dollars to engineer and build. So how does this all pertain to the Panda? The hydraulic clutch pack in the centre diff locks the front and rear axles at 50/50 because it's a direct connection with no gearing. The small amount of slip allowed before the clutches engage is helpful in this regard, but when they do the clutches are engaged to put the rear axle in motion. When that happens, and if the speed differential between the two axles is too great, either the clutches manage to lock up but the driveshaft has to deal with all that force (it can't), or the clutches slip, wear, overheat, break, and cook the hydraulic fluid (which is supposed to be a lifetime fluid). Essentially, something would break. To avoid this, the ELD system disengages over a speed that could be this damaging. With ELD off, the front wheels spin for a moment before the centre diff is engaged but traction control kicks in if the front wheels spin too much, limiting power until everything is spinning at a speed it can cope with. The Panda also lacks the power to spin the front wheels in third gear unless it's driving on snow and ice, and then due to the higher gearing, the front wheels spin too fast for the clutches to engage without damaging them unless wheel spin increases very gradually. So essentially, the 4x4 system hardly ever engages above 50kph because it the circumstances hardly every call for it, and those circumstances are hardly ever met. Even in first and second and with ELD on, if the conditions are bad enough (like extremely icy conditions), you could row through the gears and get crazy wheelspin fast enough that even though you're not going anywhere, since the wheel speed sensors and gearbox speed sensor would say you passed 50 it would disengage the rear diff because everything is spinning faster than it should or faster than it can cope with. Even so, due to the hydraulic fluid in the centre diff and the shearing forces involved, around 5% of the total torque applied is always being sent to the rear axle, and the amount of engagement from the clutch packs determines how much more can be sent. Under most circumstances, if the front is slipping it tries to send as much to the rear as it can as soon as it can, so the clutches tend to lock up rather than slip. This makes the Panda rather good at a few things: chugging over obstacles at partial throttle because even if a wheel loses contact with the ground the rest are already getting torque applied, and climbing steep hills on open throttle because it's not faffing about with wheel speed sensors trying to calculate how much torque it should send to the back because the fronts aren't enough to pull it up (this is where Haldex diffs fall behind when they're trying to be all smart about it and only sending power as needed, which makes the "torque on demand" nomenclature a bit ironic). There are, however, two things that can trip up the 4x4 system here: surfaces with so little grip that everything exceeds their design speed, and rolling roads at inspection stations.
I learned that last part the hard (and expensive) way, and also how I got to poke around in it enough to figure it out.