sm_fortressblast_action_use is a ConVar in Fortress Blast. It was added to Fortress Blast in version 0.4.
The ConVar controls what button is used to activate a powerup. This is useful for servers that are already using the default value (attack3) for something else, or game modes such as PASS Time which contains a bug where holding down attack3 may kick you from the server. The following values are accepted as buttons:
The default value of this ConVar is attack3, otherwise known as Special attack. If the value is set to something other than attack3, pressing attack3 will result in a warning message letting the player know about the change.
The instructions in the player manual will adjust for the value of this ConVar by sending the ConVar's value to the MOTD window.