Pranayama practices such as vishama-vritti are often thought of as even more powerful than asana practices. It is important when first learning pranayama to practice vishama-vritti gently and without strain.
Vishama-vritti is often described in contrast to its alternative ratio breathing technique, sama-vritti, in which the inhalation and exhalation are made to be exactly the same length. Considered a more basic pranayama, it is recommended to master sama-vritti before attempting vishama-vritti.
A common approach to vishama-vritti is to work toward a ratio of 1:4:2:1, which refers to inhaling for one count, retaining the breath in for four counts, exhaling for two counts and and retaining the breath out for one count. Retaining the breath after an exhalation is not recommended for beginners and should be worked up to gradually.