Fencing is a mechanism to isolate one node of the cluster in case it is not working properly, usually it happens when a node loses connectivity with the other. (Fencing do not happen in case of resources block or fail). Here is the way to do manual fencing.
manual fencing using ilo interface
fence_ilo -a ip-server-ilo -l admin -p password-ilo -O operator
Other way (it dipendes by the version):
fence_node_manual server_name fence_node server_name
In case of fance fail, fence_ack_node let active server works as the fail server has been fenced: