A 'true' cluster has an uneven amount of nodes (3,5,7 etc). If you want to set up a cluster with an even amount of nodes you do need a quorum disk. This is needed because otherwise it is not possible to find out the bad node(s) and a split brain situation can also arise.
True fencing is a hardware solution that can power-off one (or more) nodes if needed to protect the integrity of the data.
The manual fencing is there to provide a test environment. It is _not_ automatic (and it cannot be) and not meant to be used in production environments. Manual fencing will print a message in the log that something is wrong and a human has to take the appropriate action(s).
Here are 2 URL's that could be of interest:
Red Hat Cluster Suite
Red Hat Cluster FAQ
Hope this clears things up a bit.