It’s only used for certain shots. They shot most of the movie with a single iPhone.
What he is creating is a shifting effect where the camera moves quickly to the right and the left while the zombie is running to create this chaotic feeling while it’s sprinting.
I just hope it's not another super close up, motion sickness inducing, shaky-cam puke-fest like 28 Weeks was. I really wanted to like that movie but that style of action filming makes me car sick. I don't get how people enjoy shaky-cam movies