Its not exactly free, you have to respec twice.
Steps:
1st: Spec protection with full defiance.
2nd: Switch to defensive stance ONCE and then back to beserker stance (keep ur character sheet open, you can see the expertise change)
3rd: spec ur desired spec (not protection again ofc, AND STAY IN ZERKER STANCE)
4th: now you have 6 expertise in beserker stance.. dont go out of beserkers stance tho, you will lose the expertise if you do.
i confirmed it myself.. id post my armory profile here to prove it but im kind of paraniod.