"Kidney Punch" is a side quest available in Dying Light.
All you need to complete this side quest is find a Bomber and wait for it to kill itself, so you can search it. Infamy Bridge has bunch of them, and since they destroy all the zombies around themselves you can search them easily. Well, you have to watch out for bunch of Virals afterwards, but if you think you can pull it off...
Then go back to Dahlia and collect potion and blueprint. The potion has a side effect of flipping your camera upside down.