DattoDoesDestiny goes over how subclasses, the lock in (amplify) mechanic, and the fabled ‘third subclass’ slot works.
“Destiny will launch with two focuses per class for a total of six, but you’ll have to play through your Guardian’s progression to unlock them all.”
It’s assumed that the third slot is where your locked subclass will go.
“Destiny allows you to freely swap your ability choices, but according to Green there comes a point when you can lock them in. This gives your Guardian benefits, such as a stat boost. At this point you get a copy of the same focus, but that copy levels up much faster, thus allowing experimentation.”
When you “lock-in” you’ll have a copy of the Focus/subclass you just locked, reset back to its base, with an experience boost attached. This is done to continue to allow experimentation with a focus without needing to have to make a new character.
You still keep your old locked in subclass and can switch to it whenever you want. You can still change your play-style quickly if you want to.