entityfru traps are defined in CISCO-ENTITY-FRU-CONTROL-MIB. There are several traps. The first trap will be triggered when a module status changes, e.g. reset a module, etc. Take a look at the MIB and get familiar with each of the traps.
cefcModuleStatusChange
cefcPowerStatusChange
cefcFRUInserted
cefcFRURemoved
cefcUnrecognizedFRU
cefcFanTrayStatusChange