Correcting some Misconceptions
There are a couple of misconceptions in the question (and the ensuing discussion in comments) that I want to clear up before moving on:
Galadriel can actually do this. Well, I mean, obviously she can; she does...in the movie. But this is an exaggeration of the Jacksonverse that has limited (but not no) support from the text. As commenters have noted, this is the relevant passage:
'And now at last it comes. You will give me the Ring freely! In place of the Dark Lord you will set up a Queen. And I shall not be dark, but beautiful and terrible as the Morning and the Night! Fair as the Sea and the Sun and the Snow upon the Mountain! Dreadful as the Storm and the Lightning! Stronger than the foundations of the earth. All shall love me and despair!'
She lifted up her hand and from the ring that she wore there issued a great light that illuminated her alone and left all else dark. She stood before Frodo seeming now tall beyond measurement, and beautiful beyond enduring, terrible and worshipful. Then she let her hand fall, and the light faded, and suddenly she laughed again, and lo! she was shrunken: a slender elf-woman, clad in simple white, whose gentle voice was soft and sad.
Fellowship of the Ring Book II Chapter 7: "The Mirror of Galadriel"
So the visual effect of light and shadow is a by-product of Nenya, one of the Three Elven Rings, and not of Galadriel herself. However, the idea of her appearing more...more may be part of her own natural power. That may be supported textually, as I'll get to shortly, but Jackson hasn't clarified.
"Turbo-mode" enhances Galadriel's power. This is not remotely clear from Battle of the Five Armies. Although Galadriel does go into this state shortly before banishing Sauron, if you look closely you can see that she is also carrying the Phial of Galadriel, which she gives to Frodo in Fellowship1:
In The Two Towers we see the Phial used to great effect against Shelob:
As if [Sam's] indomitable spirit had set its potency in motion, the glass blazed suddenly like a white torch in his hand. It flamed like a star that leaping from the firmament sears the dark air with intolerable light. No such terror out of heaven had ever burned in Shelob's face before. The beams of it entered into her wounded head and scored it with unbearable pain, and the dreadful infection of light spread from eye to eye. She fell back beating the air with her forelegs, her sight blasted by inner lightnings, her mind in agony. Then turning her maimed head away, she rolled aside and began to crawl, claw by claw, towards the opening in the dark cliff behind.
The Two Towers Book IV Chapter 10: "The Choices of Master Samwise"
And in Return of the King, Sam uses it to temporarily break the power of the Watchers guarding the entrance to the Tower of Cirith Ungol:
Hardening his will Sam thrust forward once again, and halted with a jerk, staggering as if from a blow upon his breast and head. Then greatly daring, because he could think of nothing else to do, answering a sudden thought that came to him, he drew slowly out the phial of Galadriel and held it up. Its white light quickened swiftly, and the shadows under the dark arch fled. The monstrous Watchers sat there cold and still, revealed in all their hideous shape. For a moment Sam caught a glitter in the black stones of their eyes, the very malice of which made him quail; but slowly he felt their will waver and crumble into fear.
Return of the King Book VI Chapter 1: "The Tower of Cirith Ungol"
It's clear that the Phial (or, more accurately, the light of Eärendil it contains) has great inherent power over Evil. The extent to which Galadriel's greater usage of power is part of her natural abilities, and to what extent it's a product of using a magical object, is unclear.
Now that's over with, let's move on.
What is Galadriel actually doing here?
This is a question we don't really have a solid answer to, unfortunately. I'm unable to find any interview where Jackson discusses his choices in this scene, and there's almost certainly not going to be a novelization where Jackson (or Philippa Boyle) can expand further.
However, there is some precedent for this within the text. In Fellowship, Gandalf says a couple of interesting things about Glorfindel. One suggests that his wrath is "terrible", enough so that it's part of the reason the Nazgûl are defeated (emphasis mine):
'When the Ringwraiths swept by, your friends ran up behind. Close to the Ford there is a small hollow beside the road masked by a few stunted trees. There they hastily kindled fire; for Glorfindel knew that a flood would come down, if the Riders tried to cross, and then he would have to deal with any that were left on his side of the river. The moment the flood appeared, he rushed out, followed by Aragorn and the others with flaming brands. Caught between fire and water, and seeing an Elf-lord revealed in his wrath, they were dismayed, and their horses were stricken with madness. Three were carried away by the first assault of the flood; the others were now hurled into the water by their horses and overwhelmed.'
Fellowship of the Ring Book II Chapter 1: "Many Meetings"
Shortly before, he also mentions that Glorfindel glows brightly in the Unseen World:
'[The Eldar] do not fear the Ringwraiths, for those who have dwelt in the Blessed Realm live at once in both worlds, and against both the Seen and the Unseen they have great power.'
'I thought that I saw a white figure that shone and did not grow dim like the others. Was that Glorfindel then?'
'Yes, you saw him for a moment as he is upon the other side: one of the mighty of the Firstborn. He is an Elf-lord of a house of princes. Indeed there is a power in Rivendell to withstand the might of Mordor, for a while: and elsewhere other powers still dwell. There is power, too, of another kind in the Shire. But all such places will soon become islands under siege, if things go on as they are going. The Dark Lord is putting forth all his strength.'
Fellowship of the Ring Book II Chapter 1: "Many Meetings"
Either of those explanations (or a combination of both) could serve:
- "Turbo-mode" is Galadriel's wrath, revealed
- We're catching a glimpse of her in the Unseen World
1 Screenshot taken from this YouTube video of the scene