The KRC2 will only work on a 12ms cycle since that is the internal rate of the control cycle in the controller. There is no option for 4ms as there is on a KRC4.
Normally, when sending corrections to the robot they are applied during the next IPO cycle.
However, if you set the eERXFastCycle flag to TRUE then as long as you reply within 2ms the corrections that you send are used within the current IPO cycle rather than waiting for the next cycle.