Definition - What does Seiza mean?
Traditionally, seiza is one of the Japanese formal ways of sitting. It is often used in martial arts and is performed with mindfulness in both martial arts and other formal Japanese settings.
In yoga, seiza is called thunderbolt pose in English, or vajrasana in Sanskrit. The pose is often used as a resting posture for a meditative practice; although, when performed with the toes flexed and tucked under the feet, it offers a significant stretch to the feet and legs.
Yogapedia explains Seiza
Seiza encourages tranquility by clearing the mind and entering into a state of rest, which is why it is often used as a meditative pose or for pranayama and/or sadhana practice.
Outside of a yoga spiritual practice, this asana is also used by other religions as a prayerful pose. Practitioners can sit and enjoy the pose as long as is comfortable, being mindful of the breath.
Seiza can increase blood flow to the lower abdomen, which aids in digestion and pelvic health. Although it is recommended that asanas are not practiced after eating a meal, this pose is suggested to aid those with poor digestion after mealtime. It also strengthens the muscles that support good posture — the back, neck and chest — and can provide helpful relief for lower back issues and/or sciatica.