You have to keep the game console away from your eyes a bit, so you can see both screen at once. Then somewhat glance at the top screen for the flash. Once you see it, kind of, estimate which block is just fallen right when the flash happens. Destroy only blocks in the area.
By the way, do not attempt to press the screen too fast because it won't work and would ruin your rhythm. The game has somewhat a pause period between each touch that would not accept your touch from stylus. You should get used to the pause time after a while.