changes the shrinking and the position of a scrolling sprite block in its intro to create a mirror and perspective effect.
Scanline effects are special tricks using the timer interrupt to distort sprites by updating their coordinates or shrink values mid-screen.
These effects can be very CPU consuming as they can be triggered at tight intervals during active display. Consequently, they may have to use tables of precomputed values (constants, or calculated during the VBlank) to be applied fast enough.
Neo Turf Masters and Riding Hero heavily rely on this effect to render the playing field.