I don't understand what (or rather how) it does from reading this: "Knock is a message which acts like a call. When you knock a friend, your friend receives a call, and can see the text written on the screen. You get a response within 60 seconds."

- Do they get a proper phone call or an alert via the app? - If proper phone how/where does it display stuff on the screen? - What guarantees that I get a response within 60 seconds?

I would consider using images for each of the steps above rather than text.

Update: just seen the animation below which makes a few points above clearer. This should be above the explainer text paragraph (which could still do with sharpening) rather than below imo.