Uddiyana (meaning "to fly up"), or abdominal lift, is connected with the activity in the pelvis and carries the energy up much higher.
Uddiyana comes in several versions. The basic version is used with yoni mudra. When we are retaining the breath during yoni mudra, we gently lift our diaphragm a little and pull our belly in. This ties in with mulabandha coming up from the anal sphincter through the pelvis. Uddiyana does wonders as we are sitting in siddhasana too. Because we are full of air in yoni mudra kumbhaka, we will not be pulling up with the diaphragm very much. Just a little.
Basic practices should be learned before uddiyana, for it to be effective. Begin by clicking "Main Lessons" above or read more in the AYP lessons:
Uddiyana with yoni mudra - Lesson 91, lower part of the page
Uddiyana without yoni mudra, becoming dynamic and transitioning into nauli - Lesson 129
There are also a couple references in the Topic Index