Chicago Symphony Orchestra performing Mahler's "Symphony No. 6 In A Minor" Conducted by Jaap Van Zweden     Though even Mahler himself admitted that his Sixth "seems to be yet another hard nut" for critics and audience to crack, conductor Jaap Van Zweden held this nut in a vice-like grip for an uninterrupted 80 minutes at Ravinia this past Saturday.  A four-piece juggernaut of beautiful yet tragic music, Mahler's Sixth is not for the faint of heart.  It is considered by many in the classical realm, including Mahler's own wife, to be a prophetic autobiography--and if you know anything about Mahler's life after he wrote the piece, from his forced resignation from the Vienna Opera to the premature death of his daughter, you will understand the wrenching ride this symphony takes you on.     Three of the four movements are comprised of apocalyptic marches and drastic mood and tempo swings, with only fleeting moments of calm and beauty. The only rest we get is the more reserved third movement, the andante, before being thrown back to the fire that is the final movement.  The last movement is especially notable for featuring a giant hammer striking three times, the "blows of Fate" that fell our musical hero, before concluding in an agonizing pit of despair, a departure from Mahler's usual glorious finales.   Known to steer even the most disciplined conductors off the rails on occasion, Van Zweden expertly navigated the rocky path, pushing and pulling the tempo, bending the orchestra to his will to bring Mahler's story to life.  From the opening notes of the 1st movement, Van Zweden's intensity and ferocity set the tone and it was hard to take your eyes off him.  In true form, Chicago's Symphony Orchestra responded to Van Zweden's demands brilliantly, from overwhelming horn blasts to the most delicate of string melodies.  Their interpretation of this challenging piece was full of warmth and, at the same time, demonstrated the agility that the orchestra is known for.  This was not a musical experience one could soon forget and I'm sure it spawned some new Mahler fans as well, myself included.