A backdoor screwball (RHP) to a lefty slapper is downright nasty if thrown correctly!
You have to define the term "screwball". If you call a screwball a pitch that a RHP throws to a RH batter that breaks inside, then you have effectively defined a unicorn...because neither exist.
If you watched the CWS, you saw drops and rises... Pitches with the correct spin to make the ball break.
But, there were zero pitches with anything approaching the correct spin for a screwball.
In practice, a screwball is simply a "step left throw right" fastball, usually thrown with bullet spin.
The effectiveness of the pitch comes from the ball being released at a different position than the other pitches. That is, the release position of the ball is 6 inches or so further left than the batter anticipates.
So, the screwball is just an inside fastball.
Talking about throwing it at the LH batter?
Looks like hand finishes on the outside of ball