How does an epidural blood patch treat post lumbar puncture headache?

Exact mechanism unclear but here are some theories:

1. Blood injected into the epidural space may act as a plug by sealing the dural defect as it clots.
2. Increased pressure from the blood within the epidural space may stop the CSF leak.
3. Blood in the epidural space causes cephalad displacement of CSF which then relieves traction on the meninges.
   

Source

Raj: Textbook of Regional Anesthesia, 1st ed.