As you play the game, you will notice that a small meter with the word 'Zone' on it will start to fill up (it is just to the left of the playing area)
When this meter is full, just whack the
buttons at the same time. This will put you in 'the zone'.
I managed to get this in the first level on the lowest difficulty, so it should be one of the earliest achievements you get. I think I got it at around line 10.