This Bandha can be practiced both in standing posture and in sitting posture.
We will see it in standing posture.

Position of readiness
Stand with feet about a foot apart and knees slightly bent, lean forwards a little from the waist and put your hands on the knees or slightly above the knees on the thighs.
 

Steps
Inhale deeply by pushing your abdomen forwards,
and then exhaling forcibly, empty your lungs.
Contract the belly, pulling the navel and intestine towards your back. In this process diaphragm is pushed up and the wall of the belly pulled upward.
This Bandha is practices at the end of Kumbhaka (retention of breath) and beginning of Rechaka (exhalation of breath).

Second version
Do the same as above but, instead of holding your Stomach in after exhaling, rapidly push your stomach in and out ten times without taking another breath.
Stand up straight and resume normal breathing.

Benefits
Uddiyana Bandha massages the internal organs in the abdominal area, the heart.
This massaging of abdominal area improves digestive system, relieves constipation, gas, indigestion and liver trouble, tones up the nerves in the solar plexus region, reduces abdominal fat and strengthens the abdominal muscles. It also has curative and corrective effects on the adrenal glands and sex glands.
And the massaging of the heart makes it a stronger, more efficient improving the blood circulation. Regular practice of Uddiyana Bandha makes you young forever.
This Bandha is extremely helpful in following celibate. This Bandha awakens the Kundalini power.


Caution
Don't do this exercise if you have a hernia or serious heart or abdominal problems
 

Mulabandha                                    Root Lock

Uddiyana Bandha                          Upward Abdominal Lock

Jallundhar Bandha                        Chin Lock

Also read from Kundalini Yoga-SRI SWAMI SIVANANDA