The benefits of twists during a yoga practice are many: improving mobility in the spine, bringing blood flow to the digestive organs, stimulating the digestive system, detoxifying the body, strengthening the abdominals, and relieving anxiety - just to name a few.
But twisting (also known as spinal rotation) requires specific preparation before going deep.
Prior to a deep twist, the spine should be properly warmed up. The spine has six motions: flexion, extension, right side bending, left side bending, right rotation, and left rotation. All six of these motions should be incorporated in spinal warmup. Poses such as cat/cow, seated side bending, and seated rotation are a great way to gently encourage the spine to move.
The abdominals should also be activated prior to a twist. The transverse abdominis is going to protect the spine during all motions and the obliques will be the primary movers during twisting. Encouraging uddiyana bandha will activate the transverse abdominis.
During the twist, it's important to sync movement to breath. The inhale elongates the spine, encourages spinal lengthening. The exhale deepens the twist.
Also encourage students to twist from the lumbar spine (low back) first rather than the neck. They should imagine each segment rotating a small amount which results in the full rotation movement, rather than all of the rotation happening from one place. The neck is the last piece to rotate - and only if it doesn't cause discomfort.
Twists are excellent spinal neutralizers and can be used after deep backbending or forward bending to bring the spine back to neutral safely.
When should students avoid twisting? Pregnant students should avoid closed twists (any time the shoulders and hips are rotating opposite directions and towards each other) but may be okay with open twists. Some pre-existing spinal and sacroiliac joint injuries may be aggravated with twisting as well.