Thinking about a feeling tends to slow it down. Feeling it in your body tends to speed it up. Between those two you have a throttle.

What the research says

A randomised mechanistic trial in 102 combat veterans with post traumatic stress disorder deliberately separated mindfulness from slow breathing so the two could be compared, and found each engages somewhat different pathways. Different routes are not interchangeable. That trial compared those two components, so calling one a brake and the other an accelerator is clinical shorthand rather than something it measured.

Strength of evidence: one line of research. A single study, or a theoretical account.