I'm wondering whether this was a movie sin but his body position seemed to stay the same? Also why didn't he stop using Force Lightning? He continued to use it even as he was falling down.
I'm also wondering whether Palpatine meant to electrocute Vader or was it just a coincidence (because he kept firing Force Lightning) .
It makes no sense as he just stood there allowing Vader to throw him.
The Official Novelisation covers this event in a little more detail;
He did struggle, it's just that Vader is uncommonly strong and his struggles were largely ineffective.
Palpatine was in extreme pain and doesn't seem to have been able to "turn off" his force-lightning. He was concentrating on the pain of being grabbed, then the shock of being hurled into the chasm.
No. Vader got shocked because he grabbed someone who was using force-lightning. There doesn't seem to have been any intent on Palpatine's part to shock Vader.
At that instant, Vader sprang up and grabbed the Emperor from behind, pinning Palpatine’s upper arms to his torso. Weaker than he’d ever been, Vader had lain still these last few minutes, focusing his every fiber of being on this one, concentrated act - the only action possible; his last, if he failed. Ignoring pain, ignoring his shame and his weaknesses, ignoring the bone-crushing noise in his head, he focused solely and sightlessly on his will - his will to defeat the evil embodied in the Emperor.
Palpatine struggled in the grip of Vader’s unfeeling embrace, his hands still shooting bolts of malign energy out in all directions. In his wild flailing, the lightning ripped across the room, tearing into Vader. The Dark Lord fell again, electric currents crackling down his helmet, over his cape, into his heart.
Darth Vader uses is force strength, and can also control the emperors mind if you really want to because of the high level force he possesses
