Parece que depende do firmware do hardware. Como a página README diz:
Most ThinkPad fans work in "levels" at the firmware interface. Level 0 stops the fan. The higher the level, the higher the fan speed, although adjacent levels often map to the same fan speed. 7 is the highest level, where the fan reaches the maximum recommended speed.
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On the X31 and X40 (and ONLY on those models), the fan speed can be controlled to a certain degree. Once the fan is running, it can be forced to run faster or slower with the following command:
echo 'speed <speed>' > /proc/acpi/ibm/fan