The Star Wars Saga mastermind in a nutshell 

As far as I understand, Palpatine did not know that with the help of the 1000 Jedi, Rey suddenly mastered this ability. Before that, she was helplessly exposed to the force lightnings. And as far as I know, when a Jedi uses this technique, a Sith Lord can no longer end the force lightnings on his own.

has it ever been confirmed they can't stop zapping? Palpy was able to stop (eventually) against MW, so I'm not so sure about that idea.

ROS is NOT CANON TO ME

As far as I know, when a Jedi uses this technique, a Sith Lord can no longer end the force lightnings on his own.

Kardanwelle1994 Yeah but is that point made at all in the movie? If it’s not established in the movie people are just gonna think Palpatine is dumb for continuing to electrocute Rey and let the lightning bounce back at him

Dies*

On the contrary, if you see clone Palpatine's hands, they stopped firing. So yes, Rey is firing Sith Lightning from the sabers like a Sith.