Straddling means the person under the gun makes a raise before the cards are dealt (or at least before he looks at them).
The raise stands, so the next person to act can call or reraise. When the action gets back around to the straddler, he can reraise again. So even if everyone just calls, he can reraise his own bet.
Some casinos do not allow straddling. They can't stop you from raising blind UTG, but if everyone calls back to you, you cannot reraise your own bet.
The point of the straddle is to create action and to gamb0000000l.
Check out the B&M sticky for a great straddle report.
I know nothing about running it twice.
Edit:
LINKY to JSD's Guide to Straddling