It is not obvious from the documentation what IPMI commands are supported by Zabbix.
It seems that only "Basic controls" are supported:

<command> <value>

Where <value> is an integer resp. "on" or "off" and <command> is (theoretically) one of these:

• relay
• alarm
• reset
• power
• fan speed
• one-shot reset
• output
• one-shot output

Since blanks are not supported for commands, commands like "fan speed" can neither be used.