Clocknapper

To steal a phase means that during that player’s next turn, as the chosen phase begins, you treat it as if it were your turn. Here’s what happens if you steal each phase:

Combat phase: Just like combat on your turn, you can attack your opponents with your creatures. The opponent you stole the phase from can’t attack with their creatures, but they can block.

Beginning phase: You untap permanents you control and your opponent doesn’t. Abilities that trigger at the beginning of your upkeep happen and ones that trigger at the beginning of their upkeep don’t. You draw a card as it’s now your draw step, so your opponent doesn’t.