The start of a hand?

[AWLurch]

You may buy chips and play behind before the cards are in the air, meaning before the first card is dealt as noted. \

As far as the "beginning of the hand" from a players standpoint I believe it is when the first card is in the air. From the dealers standpoint as far as procedure is concerned the first riffle of the deck constitutes the beginning of the shuffle and official start of the hand.

It is this period between the first riffle and first card in the air that is the grey area, but straddling, posting, moving seats and playing behind is allowed during this area.

But you should never see a dealer change the shuffling procedure after the first riffle. This is primarily a concern in tournaments. If the dealer has riffled once before the tournament director announces a changing of a level, the hand shall be played on the previous level and change the next hand. This is atleast the policy where I deal, cash games of course allow for the grey area to come into play.