Yes when the current shares drop down to 0 or a low number it means a shift ended and a new one started. If the open shift earnings doesn't change it means we're having bad luck (a lot of that the last few days) an did not find a block in the last shift.
But when I look back at each shift, none of them has 0 blocks, currently around 15 blocks each shift. And there is a statement:
"Open shifts continue to receive payments until 10 additional shifts have completed."
When a shift is completed, the following will happen on your PPLNS Stat summary:
1) Shares in current shift will reset to 0 (you probably won't refresh the exact moment it resets to 0, but you will load at a lower number)
2a) Shares in open shifts will increase by the shares you had in the current shift (now the newest open shift)
2b) Shares in open shifts decreases by the number of shares you had in the oldest open shift (now closed)
3a) Open shift earnings will decrease by the amount paid to the shift that was just closed.
3b) Closed shift earnings will increase by the amount that was in that shift.