Topic rounds it up in one sentence.
But for those who needs it over-explained:
Whenever multicast would prock, it gives you additional casts equal to the number displayed. For example, Multicast grants 2 additional casts on prock at skill level 1 of Multicast. Making it a total of 3 casts with yours included that procked in the first place.
This is not stated in the tooltips but is the actual outcome. Correct either the number of additional spells casted on prock or the tooltips.
Report based on latest version as of V3a.