What’s the bonus or idea of getting a ball trapped behind the shield?

I have not did this myself but form the rules page, also get one ball out of the way in multi ball safe from draining.

A useful trick while in multiball is to try and get a ball trapped behind the forcefield. Then when another ball hits the front of the forcefield the trapped ball will (usually) bounce straight up into the hole behind the drop target. This awards you the points for the next city and displays the words "Dirty Pool" on the DMD

http://pinball.org/rules/attackfrommars3.html

You then hit the shield which acts like a captive ball of sorts.. then the ball goes in the hole and awards you dirty pool and a country finished also.