Fire Spin indeeds traps opposing pokemon but when Blaziken switches out, this is canceled, meaning your opponent is no longer trapped and that he can switch out again. This can be solved however with the move Baton Pass, which Blaziken is able to learn.
Hey thanks, I only really used Will-O-Wisp competitively. I used Fire Spin once, I guess the opponent just decided not to switch out. :/ ?
I'll have to test it more now anyways. It would be similiar to Umbreon's passing except Blaziken is faster so he can pass before getting hit. Fire Spin only has a 37.5% chance of trapping only 2 turns so set up moves are useable, or you could just kill the foe right then and there.
Edit: Will-O-Wisp has always been very useful though and I could replace the Fire Spin slot for another attack.