As the monitor is drawing a frame, it will include the newer information in the lower half of the screen. Thus, you will get a slice of the new frame.
If the monitor's refresh rate were higher, it would draw the entire new frame, but if the monitor's refresh rate is slower, then it only has time to draw a slice of the new frame.
But at least you get a slice. Who knows: it might be the slice that has the incoming rocket in it.