You're right in that the cracks do already exist. When they enhance them with high pressure they open the cracks up. Due to natural tensions in the rocks when the cracks are opened they slip. Then when the pressure is released and the cracks come together again they no longer meet up as before. The rock is left with gaps and path ways for the brine to flow through. As I understand it this is a technique used in the oil industry to improve oil flow.
Cheers
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