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[Peter Jackson's LotR Adaptation] When Isildur cuts the ring from Sauron's finger, Sauron 'explodes' and everyone around him is knocked over by the shockwave. What is actually happening here?

My question basically boils down to: Why does Sauron explode?

He lights up, like he's suddenly overflowing with power. Is this risidual power from the ring that is now too much to be contained without it? 

But the blast seems physical in nature seeing how it affects those near. The ring's power is never seen as physical at any other time, why now?

So instead maybe it's more magical in nature beyond just the ring? Is it his 'spirit' escaping? Or perhaps the blast is a result of another power intervening to protect Middle Earth (by straight up exploding Sauron) now that he has been weakened enough. The Valar?

(This is in reference to the PJ Fellowship prologue, I do not know if Sauron explodes like this in the book but in universe answers from both are appreciated!)
gpt
Sauron used to be a shapeshifter whose go-to MO was assuming a very "fair" form and tricking people into trusting him and then bending them to his will. This is the trick he used to turn the men of Numenor evil and use them to assault the abode of the gods (and elves) in the second age. When they intervened and sank Numenor, his corporeal form was lost and he only escaped as a spirit. 

Luckily for him, he had put of a lot of his power into his ring and he could use that to bind himself into a human-ish shape. This is what you see at the start of the film. When Isildur cuts off his ring, all the energy used to maintain that form must've escaped violently, causing the explosion. It took centuries for him to gather himself together but without the ring the best he could do is form the eye that you see in the trilogy. 

In the book it's a bit different. He isn't beaten by fluke, the alliance had laid siege to Barad-Ur for many years and he only enters the fight as a last resort. The king of the elves and Elendil (Isildur's father) take him on and defeat him in combat at the cost of their lives. Isildur then uses Elendil's sword (that was broken in the fight) to cut the ring off Sauron's downed body. His spirit escapes, but not as dramatically in the film.