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This item causes the move Techno Blast to become Ice-type. It does not change it in any other way, however, and has no effect on a Pokemon without this move.
Only two Pokemon in the game can use Techno Blast, and the only one who can actually make use of it is Genesect. However, you really shouldn't be using this item even on Genesect, for a number of reasons. For one thing, holding Chill Drive means that Genesect cannot then hold a boosting item, and will miss the kick from Life Orb or Choice Scarf dreadfully. Furthermore, Genesect has access to Ice Beam, which is more powerful than Ice-type Techno Blast regardless, and does allow you to use Life Orb. Like the other Drive items, you should never use Chill Drive competitively. It is just not worth it.
The Chill Drive is received in Pokemon White if you show Genesect to the Scientist inside the P2 Laboratory on Route 17. It is not obtainable in Pokemon Black.