Separate the 3 pieces, and assemble them again. 

"I found a smaller version of this puzzle, patented in America in 1975 by Eldon Vaughn, about twenty years ago, but considered it impossible to solve at the time.
We have developed a new version of this puzzle, which retains the difficulty of the original, and licensed it for the Cast Puzzle series.
This is without a doubt among the most difficult of all the puzzles.
The key word is 'Twist.
Don’t use too much force…the pieces should just slide apart."

-NOB, Cast designer