I was wondering what the effects are and if you are even suppose to do it in some practices, doing Uddiyana Bandha, and while you're navel is in and upwards the spine, you continue to inhale (while still holding Uddiyana Bandha) so the energy reaches the head? are you suppose to do such a thing? because when you inhale while holding Uddiyana Bandha it's hard to inhale and you cant really get enough breath for the energy to reach the head..
I think uddiyana is best done with the air expelled out of your lungs, during asanas.
To do navi kriya (which is similar) during practices like spinal breathing and yoni mudra kumbhaka this lesson should help http://www.aypsite.org/275.html
With both of these practices you are guiding the kundalini up. Be careful because too much can lead to overload.
A gentle uddiyana can be used while retaining the breath. The sensation is that while the lungs are full that the movement of the energy of inhalation continues in the ribs which creates a natural uddiyana