In Legends of Tomorrow Season 2 Episode 10 "The Legion of Doom", Eobard Thawne is trapped inside of the Swiss bank vault with Damien Dahrk and Malcom Merlyn, and pleads with them to open the door so that they won't be trapped by the thing that is hunting him.
Why didn't he simply phase through the walls? It doesn't seem like the bank vault is lined with the sort of force-field that has been used to trap speedsters.
Before the vault door fully closed on Black Flash, a grid of orange beams appeared, seemingly preventing his escape. Although the beams did not appear when Malcolm closed the door on Eobard earlier, Eobard was close to panic at the time and may not have noticed their absence.
If the walls, floor, and ceiling of the vault are made of metal like the door, then the vault is a Faraday cage. Since Eobard's powers have electrical properties, a metal enclosure may be an effective passive barrier to phasing.
