The fastest way to hack your brain into parasympathetic mode is the supported pelvic rest pose — a bolster under your pelvis and legs.
When you're tense, the nervous system keeps deep pelvic and back muscles braced as a protection reflex. This setup removes gravitational demand completely. The brain registers support and almost zero effort.
You may feel involuntary contractions in your lower spine during the first couple of minutes — this is normal. Tune into those sensations and let those muscles release.
This pose tells your nervous system: nothing is required right now.