Savage Beating
Instant
Cast this spell only during combat on your turn.
Choose one —
• Creatures you control gain double strike until end of turn.
• Untap all creatures you control. After this phase, there is an additional combat phase.
Entwine 
(Choose both if you pay the entwine cost.)
Matt Thompson
Commander Masters (CMM)
• #252Rulings
The additional combat phase is directly after the combat phase in which Savage Beating is cast. There isn't a main phase between the two combat phases.
(wotc • 2004-12-01)
Rulings
The additional combat phase is directly after the combat phase in which Savage Beating is cast. There isn't a main phase between the two combat phases.
(wotc • 2004-12-01)