Shield inverter
The shield inverter was part of the deflector dish aboard Federation starships like the .

By activating the shield inverter, the direction in which the deflector shield works can be reversed, thus a pull-movement can be turned into a push-movement. 

The shield inverter of the Enterprise-D was activated in 2368 after the deflector dish was used to absorb high-energy plasma from the atmosphere of Penthara IV. The plasma was released into space after activating the inverter. ()
Shield technology
Shield technology