Hi there!
Steem Power represents your influence when voting, the more Steem Power or SP the more your vote is worth to others and even yourself. But you can't withdraw SP right away, when you Power Down some of it is converted to STEEM (which you can withdraw right away) weekly and after three months you'd have it all.
When you do 100% then you are choosing your payout to be 100% in SP which gives you more influence, but if you go with 50/50 then half of it would be SP and the other half would be SBD aka Steem Dollars.
Steem Dollars can be withdrawn right away, from the moment the balance is shown in your wallet. That's the positive in choosing the 50/50 type of payout.
Let me know if you have any questions and welcome to Steemit.
RE: Im new to Steemit and need your help with a feature..