It is not concerned with hand ar arm because it is a phrasal Verb which means - To deliver (an indictment) to a higher judicial authority.

When teachers address a classroom of students, and they want to take a poll to see what the students think, they might say: "Hands up if you already read chapter one!" which tells the children to raise their hands if they read that chapter. The teacher might also say "Raise your hand if you read chapter one." ("Hands up" is a command, and "raise your hand" is a request.) A bank robber might say "hands up!" to someone that he is holding at gunpoint, so he can see their hands and know they are not reaching for a weapon. In an office setting, the boss might say "who is presenting their findings at tomorrow's meeting? Hands up!" and then look around the room for volunteers. School children usually raise their hands by shooting their arm straight up in the air, with their hand as high as possible, but only raise one hand. Adults often raise their hands in a more subtle way, like lifting the hand part way up, or raising their index finger, or holding up their pen in the air if in a meeting, but again they only raise one hand. In the bank robber scenario, the victim would have "hands up" by lifting both hands, palms forward, either straight up or in line with their head. (One would never say "hand up" with the singular "hand.")