A voting disk is a backup communications mechanism that allows CSS daemons to negotiate which sub-cluster will survive. These voting disks keep a status of who is currently alive and counts votes in case of a cluster reconfiguration. It works as follows:
a) Ensures that you cannot join the cluster if you cannot access the voting disk(s)
b) Leave the cluster if you cannot communicate with it (to ensure we do not have aberrant nodes)
c) Should multiple sub-clusters form, it will only allow one to continue. It prefers a greater number of nodes, and secondly the node with the lowest incarnation number.
d) Is kept redundant by Oracle in 10g Release 2 (you need to access a majority of existing voting disks)
At most only one sub-cluster will continue and a split brain will be avoided.