In simple terms, Powering up, keeps your rewards in the Steemit community until you decide to power down (which takes 13 weeks too do with 7 day installments back into STEEM which you can trade on an exchange into BTC or other. If you choose to keep 50/50 then half will be going into SBD (savings) , and half into STEEM . One benefit for you of Powering up 100% is that it increases your voting power, this is good to have. You can choose for each post, so you could have a mix of both. Try posting with 100% power up for a day then next day go with 50/50...see how that works for you.
RE: Power up 100% or Default 50% 50%. What does it mean.