"Engage the core..."

An abdominal lock is the pulling in of the muscles above and below the navel. The full lock (Uddiyana Bandha) should only be done on an empty stomach. But in our practice, we would only carry out a partial lock, which involves contracting those muscles on the end of the inhalation as the exhalation begins.

Its purpose is to stabilise the core and protect the spine in our asanas.