Thanks to @papa-pepper I finally get it, I think.
Internally, SBD is tied to $1 USD. Another way of saying it is SBD <= $1 USD even though the exchanges price may be greater than $1.
If the value of SBD at the exchanges is > $1.00 use 100% power up when you create a new post.
If the value of SBD <= $1.00 use 50/50 power up when you create a new post.
Here's why.
Situation: current value of SBD on the market is $2. You get a payout of $2.
Choosing 100% causes your $2 payout to go to Steem. Nothing is left in SBD.
Choosing 50/50 causes your $2 payout to be split with $1 going to Steem and $1 staying in SBD. Now you can cash out your $1 SBD at the exchange for $2. You just made and extra $1 because of the exchange rate being greater than $1. Of course you don't have to cash out. You can leave that $1 in SBD and cash out later at whatever the market rate is at that time.
Please check my work. Did I get this right?
Update: I went through this explanation and then violated the rules SBD was at $.94 and I powered up 100%. Geez. What should I have done?