What is the sound effect called from ~2:45-2:46 in the Nicki Minaj - High School video?
That is simply a backwards reverb effect - added post production. To get it you used to play the recording backwards through the reverb unit, record the output, and then play the track forwards again.
With a modern DAW, it is as easy as selecting the section of the track you wish to use backwards reverb on and set the parameters. In this case the mix is quite mild, and the reverb time pretty short.