Power struggle is when a child refuses to do something you let them do then argument and temper flares carries on. Kids won't easily back down in a confrontation as they see this as an opportunity to gain power. Sometimes the more desperate a parent becomes to get a child to comply, the more resistant the child grows. So the very first step for parents, is to be willing to let go to avoid the situation to heat up.
Then think of ways to give them power in a good way so they're not grabbing the power inappropriately. Think of a solution you can both agree in that situation. Offer the kids choices. Kids loves choices. Choices give power, it gives the kids a sense of control. Let them join you in making the decision to choose to make them feel empowered.
Choices are very important. When possible, offer two choices that are 100% totally acceptable to you and either one they choose is absolutely fine.
Language is also very important. You need to use words that really do indicate a choice. Avoid threatening or bribing when offering choices. Lastly, state your expectations clearly and make your requests calmly. Make your statement short but sweet.