The phenomenon that nearly ALWAYS occurs while playing poker - usually Texas No-Limit Hold'em - at the final table in online pokersites where the shortstack (almost out of the game) and the bigstack (chip leader) positions swap.
This usually will happen with only 4 or 5 seats left in the game and quite often results with the former bigstack knocked out of the game in the 4th or 5th position while the former shortstack will go on to take 2nd position or even the WIN.
In a sit and go, a Chipstack Reversal is usually directly associated with the 'bubble' - last unpaid seat in a tournament/sit and go.
'You're the chipleader on the bubble, Freddie. Watch out for the Chipstack Reversal.'