I'll share my own impression of two intsances of the 'flow' I've personally experienced:
First is indeed like a trance, when I cease to be aware of time at all and most striking instance ocurred (I vividly remember the feeling) when I've been reading RoTK after waiting for it for two years (the story in short - I did not know English back than, and between publication of translations of Books 1-4 and 5-6 approximately two years have passed).
The 'flow' associated with labour evokes different kind of feeling in me (and I have been having more of it recently than I've bargained for) - it usually occurs when I'm approaching a deadline and feel that time is short and workload on my hands is bigger than the timespan left may possibly contain. Than I'm not 'unaware' of time passage, on the contrary, I'm more than sharply aware of it, I feel hours rushing by with my very skin. Though, I must admit, I work better under stress, as I'm more focused and set to the task and do not digress (as I do now, per instance, posting here instead of doing the job - but now the deadline I should hand over the work is still far off for me to worry
)