A lot of this has been posted before.
If you know it is and you willingly take it, you take sides with Satan to complete his grand work. Your willing allegiance, shown either through your thoughts and will (the forehead) or through your actions (your hand)
You want to go further? The injection into the arm is good enough. The bible is written in ancient greek. The greek word for hand used was chier (χέρι)
This word essentially means any part of your arm from your shoulder to your hand, and this usage can be found within Homer's Odyssey
However, the word for forehead (metapon, μέτωπον) essentially means above or beyond the eyes, but this also describes where the brain is, and thus describes where the brain is, so this is your will or thoughts.
That is the mark.