Knock Your iPhone to Unlock Your Mac

This is an interesting concept (and from what I’ve read it works well) but I have a few questions:

• does the app need to have been launched once beforehand? I suppose so (how else would it work?) but, hey, you never know know…

• what if the app is closed?

• the Mac app doesn’t use the genuine lockscreen, but a screen of its own: what are the limitations to it? Can I unlock my Mac through the app when it boots up?

Alas, four bucks seems like a good deal for such a convenient app duo.