Beaters have two roles on a Quidditch team:
1. To hit the Bludgers towards players of the opposing team.
2. To protect members of their team from Bludgers knocked towards them.

+beat [Bludger1 or Bludger2]
   This will send the specified Bludger away from one of your players. It is to be used when you are defending one of your team members. If you want to try and attack another player with the bludger, use the next command.
(Available in game and practice code)

+beat [Bludger1 or Bludger2] at [name]
   This command is used to attack with a Bludger. A Beater has a chance of successfully hitting the Bludger and aiming it correctly, hitting the Bludger and missing his target, or missing the Bludger completely. You may attempt to defend a team member and attack a player on the opposing team at the same time. In order to do this, choose a bludger that is currently targetting one of your team members (as opposed to one that is still looking for a target). Please note that if you are trying to defend AND attack at the same time, your chances of success drop significantly.
(Available in game and practice code)

If you are hit with a bludger:
Grazed: Miss one turn.
Hit hard: Miss two turns. Three hard hits knocks you off of your broomstick.
Knocked off broom: You are out of the game.

Note on Bludgers:
The full game code will actually keep track of where two Bludgers are at all times. Therefore, it is necessary for Beaters to specify which one they are pursuing (bludger1 or bludger2). If the Beaters do not actively beat at the Bludgers, they will randomly hone in on a player. It is also assumed that each player is doing his best to dodge whilst playing. Therefore, there is no command required in the full game code to dodge.

+dodge
   Using this command will test your abilities to dodge against an oncoming Bludger.
(Practice code only - NOT available during games)

+skip turn
   Use this on your turn if you have been hit with the bludger (two rounds in a row if you are hit hard). The code will not permit you to execute any other commands until your number of missed turns are up.
(Full game only - NOT available during practice)

For your reference...
All three of these are only available to you during games.

+lineup
   During a full game, this command will bring up the player lineup for the two teams that are currently competing. It also lists their positions. If you have ANSI colours turned on, players who are actually connected will have purple names.

+qhelp
   For those players struggling to remember which commands are available to them during a full game, typing this command will bring up a short list of what they can use, along with a short description of the command.

+report
   During a full game, this will bring up a list of statistics for the game in progress. Basically, it gives a report of who has the Quaffle, where the Bludgers are on the field and who has yet to take a turn, amongst other things.