Murcha means "retain".
In this technique practitioners hold the breath until consciousness slightly alters.
Many time students are encouraged to hold for as long as possible or until you feel faint but it is always best to do your breathing techniques sensibly and compassionately so perhaps you want to do it gently and exhale well before you feel an intense desire to let the air out or faint...
Murchha is great way to measure your progress in pranayama. For instance if one performs this technique by holding at the beginning of a pranayama session the time one can retain will be much shorter than when one performs it at the end. This shows the practitioner that the exchange of air and prana has substantially improved.
Knowing how long you can safely hold the air in your lungs is essential in pranayama.
Find a comfortable seated position. Gently exhale all of the air from the lungs.
Inhale and drop the chin to the chest if it is not already there.
Hold for as long as possible and count how long you can hold.
It's hard to slowly exhale if you hold too long so stop a
little short so that you don't have to exhale with such haste.
After talking some normal breaths once again try to retain the breath. This time try a little longer.