opjose

The "screw" is a small pot ( Potentiometer ).

It varies the resistance to the the electronics.

Yes it is used to center the two servos.

Frankly when I've used this type of setup I've found that you must adjust the pot at every trip to the field.

There are some servo reversers ( more expensive types... ) that handle this in a digital manner, and instead of a pot, they feature a digital switch instead.

I've tried what you have and disliked it.

I have not tried the more expensive variety.