Polo Rules

Where's the line with this? When does controlling the ball along the boards become pinning?

Holding a player against the boards (when they don't have the ball) is trapping (used to be called pinning) and isn't allowed, it's usually called if the player on the outside fails to move away immediately after taking someone to the boards.

Holding the ball at the boards is a delay of game penalty and is usually called if the player isn't actively trying to get the ball back into play (sometimes it's obvious, sometimes not).

There is a slight grey area where a player can trap (pin) someone against the boards if the ball is there (as they're waiting for the ball to come out) and when the player on the board is failing to get the ball back into play (because they might feel like they're being trapped, etc). In this case the only potential calls is a delay of game penalty on the ball handler (as trapping specifically excludes incidents when the ball is present), but most of the time nothing is called in this situation, go figure.

§3.3.3 – The referee establishes both teams are ready by receiving a verbal acknowledgment.

This is going to be removed in the next edit (or seriously revised) as it's never been used/called and is a neat way to try and disallow a "cheap shot" if the ref is inexperienced.