From wiki: http://en.wikipedia.org/wiki/Flow_(psychology)
Flow is the mental state of operation in which the person is fully immersed in what he or she is doing, characterized by a feeling of energized focus, full involvement, and success in the process of the activity. Proposed by psychologist Mihály Csíkszentmihályi, the concept has been widely referenced across a variety of fields.
Colloquial terms for this or similar mental states include: to be on the ball, in the zone, or in the groove.
Really interesting phenomena - and great when it happens.
The first time I remember that feeling was playing Galaga when I was very young - the minute I realized I was doing something different it was over - I lost all my lives in short fashion after that.
Used to happen a lot playing Enduro as well.