In contrast to sportsmanship, where players are fair, equitable and play within the rules, gamesmanship describes a player who bends the rules so they are not breaking the rules, but certainly not playing fairly.
A player who plays with gamesmanship will tackle too hard- bordering being illegal; will not offer a hand to a fallen player; will confront a player intimidating them with close physical presence and intense eye contact; or picking a route that will cut the opposition up.
Gamesmanship is often used to gain an unfair advantage over a team or player, by intimidating them or making them angry, which will affect their ability to cope with stress or pressure, thus affecting their arousal and drive which could result in a negative effect on performance.
Referees are often powerless to take action on gamesmanship, as the player bends the rules so they do not break them. Until a player oversteps the line, no disciplinary action can be taken.