Supposedly, When Prime grabs his swords he has quite the struggle picking it up similar to Arthur and Excaliber. And apperantly it's been cut from a few showings that the sword contains the souls of ancient Cybertronian Knights.
I'm fairly certain the sword had nothing to do with it. The plates around his arms "harden" when he's pulling up the sword, but I'm pretty sure that was meant to be him strengthening his arms because it was stuck in the stone so hard. The sudden flying was pretty weird though! But, eh, Bayfilm. Explanations never happen.
This is the one thing I was hinting at that the bay haters would probably jump on bay over. Why out of nowhere is OP able to just fly from boosters and never once did he in the past 3 movies. And while I still think it could have been explained a bit better so this would not be questioned, he gets the feet booster upgrade when he gets his Silver Knight upgrade as he gains the sword. It's part of his Silve Knight abilities.
He appears to have boosters built into his back that fold out when he's flying. The whole thing is very Iron Man-esque.
But they just appear, right? Like they have no explanation? This is gonna be amazingly laughable, best joke of the whole movie.
Yep, they just appear. He kills Lockdown and is just like "I'm outta here, bitches!" and rockets off with Galvatron watching in the distance. He then lands, talks to Cade and the Autobots before rocketting off again straight into space. It's... Bizarre.
There is a whole scene that was cut. When OP grabs the sword, he is sort of transported into a VR world where he meets the past knights that died and owned the ship, and surprise, they look a lot like the Dinobots They give OP his Silver knight abilities and his armor changes silver and his arms and legs change into the new armor type and he gets the booster upgraded. But the whole VR scene was cut. We just get OP grabbing the sword and his armor changing but not into the silver concept like in the VR scene.
How do we know about this scene? Honestly, it sounds kinda suspect. The scene didn't really leave a moment for the Knights of Cybertron to appear and Optimus certainly wasn't being upgraded in his arms. Hardening his arms to do things is not a new ability of Prime's, he did the same thing when killing Shockwave. There were no visible differences in Optimus design once he had the sword at all.
Also, remember the first cover pic that was pointed out on the Empire Mag of OP not having his stacks on his back and looked to have boosters?