Imo, saying arrows make players motivated (like people do when talking about formations) is a good way of describing it. I don't know much, but I believe a red arrow means the player will attack more readily, get forward faster and will lose more condition while attacking than if not motivated. He will defend as normal - this can however mean that if he is advanced then sometimes he might not track back quick enough, but I believe he'll still try. A player with blue arrow will similarly get back more readily, when the opposition attacks, again for a greater condition cost than if 'un-arrowed'. He will still attack while his team attacks - its just he won't do it as readily and he may be doing so from a more withdrawn position if the change from defence to attack is quick.

Just my thoughts, my beliefs.