how do jewelers "work harden" platinum

When molten metal solidifies, it does so in a pattern of small crystals, with tiny spaces between them. When the metal is worked (shaped with a hammer, for example), these crystals become deformed and spread out, taking over the empty spaces. This causes the metal to become harder and more brittle. If the metal is overworked, the crystals become so stressed that the metal can crack.

To restore the metal to working conditions, it is annealed: it is heated up to a specific temperature (according to the metal or alloy) and then cooled. Some metal are quickly cooled - or quenched - in water or alcohol (silver, for example) while others must be aircooled (rose gold can shatter if it is quenched). This process reorders the metal, reorganizing the crystals and recreating the open spaces.

A fabricated piece is naturally workhardened during the fabrication process. I imagine you are refering to a cast piece, then. Tumbling a piece in shot will workharden it slightly (but I am not sure it works on platinum). However, for significant workhardening, the metal must be deformed. I don''t see how this is possible in a cast piece (pros, please correct me if I am wrong).