Although not convinced that what Reiki describes (and yes there is a feeling there) is a 'chi ball' there is an increase of blood flow and heat to the hands
I've got two problems with this theory:
1) I know how increased bloodflow feels and it isn't the same
2) you can feel qi although your hands are cold (which they are quite frequently in my part of the world

)
anybody come up with a more convincing theory?