To hold or lock is a bandh. The bandhas lock the prana in particular areas and redirect their flow through different nadis.  Bandhas are the neuromuscular locks. In yoga we have four  locks- Jalandhar bandh(throat lock) Uddiyan bandh(abdominal lock) and Mool bandh (perineum lock) and Maha Bandh(mega lock). While doing these bandhas we give the  pressure on some glands. As glands are porous in nature the  secretion of hormones help the different system of the body to function well.  Like performing the Jaalandhar bandh( throat lock) we stimulate the thyroid gland. The thyroxin is released in this lock. But if a person  already has over secretion of thyroxin it is not good for them .