Rule 500.10 · General
Some effects add a step after a particular phase. In that case, that effect first creates the phase which normally contains that step directly after the specified phase. Any other steps that phase would normally have are skipped (see rule 500.11).
Example: Obeka, Splitter of Seconds says, in part, “Whenever Obeka deals combat damage to a player, you get that many additional upkeep steps after this phase.” After that ability resolves, its controller adds that many beginning phases after this phase. Those new beginning phases have only an upkeep step. The untap steps and draw steps of those phases are skipped.
Subrules
If an effect that says “you get” an additional step or phase would add a step or phase to a turn other than its controller’s, no steps or phases are added.