Its not really a secret but yh.first u have to lose a lot of hp until ur hp is red and then get full chakra....(i use with nunchuck and wii remote)then click down on ur nunchuck and down on ur wii remote.then u will be in ninetail mode. This is wat happes wen u get nintail mode.Ur strong attack is diffent, ur special is different, the b moves are different, u get some more hp.
p.s. for the pple who thought this was ninetail beserk mode it tells u how on ur menu on page 8.look for naruto's name and read what they all say. btw u can do rasengan on the enemy in a chrises