Since doing push-ups to infinity can get monotonous, it's always nice to add a bit of variety. This is where this challenge comes in.
Don't use any weight for the shoulder press. The goal here is to complete as many reps as possible, and adding weight will fatigue you faster which makes it far less grueling than actually performing the press using just your arms as weight.
Any challenge that encourages you to do as many reps as possible of a certain exercise is obviously going to be difficult. The kicker with this exercise, however, is you get a short break between the push-ups and the arm raises which allows you to recover just enough to perform your next round.
This makes you question whether or not you can do another set – which adds a mental toughness component to an already incredibly difficult physical challenge.