I'll try!
To drive on open roads around corners, the cars wheels need to be able to turn at different rates.
Image turning to the left in a tight circle or arc, the inside wheels (left) only need to rotate once, but the outside (right) need to turn four or five times to keep up as they have a greater distance to cover in the arc.
For the car to do this, the diff is open or infinite slip diff. (some sports cars have limited slip diffs to help corner under power, but they both act similar when in mud)
It works by magic (or maybe I'm not clever enough to explain) but what it means is if one wheel is unloaded (or slips) the diff will not send power to the other wheel on that axle, it just sits there dead.
On slippy surfaces this open diff is a problem because as soon as one wheel spins in the mud, the other has no chance of powering the car as the diff will not send power to it.
One wheel will just spin away like mad, the other will do feck all!
To get around this problem and HELP with grip, some way of locking the diff is needed, so it doesn't matter if one wheel slips, the other will still get power and perhaps have a chance of finding some grip and moving the car forward.
Obviously you can't run a locked diff on roads with plenty of grip as things like half shafts snap at the first corner you try driving around. (I've tried it in an old Disco!)
Old 4x4 used big heavy metal cogs in separate gearboxes or hydraulics to force the diff to lock together the two half shafts on that axle and connecting both wheels/half shafts together (and perhaps lock the front axle's rotation to the rear as well so all four wheels are locked together)
Traction + and ELD doesn't use these heavy parts, but use the on board electronics like ABS and ESP so the braking system loosely simulates a locked diff.
The diff is a traditional open diff (with the same shortfalls explained above) but it detects a slipping wheel via the ABS wheel speed sensor and applies that wheels brake, thus sending power to the wheel on the other axle.
What you must remember is if the diff is powering one, two, three or four wheels doesn't mean any will grip the ground, but the more being powered, the more chance!
Using the brakes on a slipping wheel only helps if the other wheel(s) grip, but grip or not, it will mean the engine power will find it's way to the other wheel on that axle where before it wouldn't.
Spin both (or all four with a 4x4) and it's not going to work, it can't brake both (or all 4) and hope to regain grip.
It's not just this system, but the old style physical diff locks would also suffer the same, spinning all wheels in the mud isn't going to get you moving forward.
The same can be said about ABS (and was in another recent post)
The wheel speed sensors detect each wheels speed, if one, two or three rotate slower, it quickly releases and re applies them until they start rotating at the same speed.
Slam the brakes on hard on a slippy surface (ice or snow) and lock all four up at the same time and the ABS system is useless, how can it work? It cannot detect a difference of rotation speed at any wheel, it presumes you're stopped so you slide and crash!
Best way to try and get the car moving on slippy surfaces even with ELD, Traction + or any other device is to lift the clutch gently without any throttle pedal.
Just put it in first, release the hand brake and lift the clutch very gently.
The ECU will automatically add enough rpm to prevent a stall and you will creep forward, hopefully without the wheel spin.