As you know, the addition of vCenter 5.1 added a few . . . well . . . major things: Single Sign-on or SSO, the Web Client, a plethora of new interface views, and so forth. One such new view is pictured below.
When vSphere HA is enabled for a cluster, all active hosts (those not in standby or maintenance mode, or not disconnected) participate in an election to choose the cluster's master host. The host that mounts the greatest number of datastores has an advantage in the election. Only one master host typically exists per cluster and all other hosts are slave hosts. If the master host fails, is shut down or put in standby mode, or isremoved from the cluster a new election is held. — vSphere Availability Guide
If you would like to force a certain host to always be elected the master anytime cluster HA operations are reconfigured, you can also accomplish this by configuring Advanced Options on the ESXi host in question. (Use with extreme caution)
When a master election is held, the FDMs exchange a goodness value, and the FDM with the largest goodness value is elected master. Ties are broken using the host IDs assigned by vCenter Server. This parameter can be used to override the computed goodness value for a given FDM. To force a specific host to be elected master each time an election is held and the host is active, set this option to a large positive value. This option should not be specified at the cluster level. — VMware KB 2033250