Hi!
In Spanish the conjugation of the verb tells you whose hand or eyes the person is referring to. If I say "levanta las manos", the verb "levanta" is conjugated in the second person singular (tú) so it's obvious that "tú" is the one who needs to raise the hands.
The only time we use the possessives is when the action does not relate to the person you're talking to. For example, you have my book. In that case I tell you "dame MI libro", because if I say "el libro" it doesn't make clear which book I'm talking about.
I don't know if this explanation makes sense, this is my very first reply in this forum.
