Understanding LiPo battery gas gauge circuit
The datasheet says this about the "PACK" (2) pin:


  Battery pack input voltage sense input. It also serves as device wake up when the device is in shutdown mode 


It is also marked as "analog input" and "power".

So, when you connect the charger, the chip gets power from the PACK pin then it turns on the relevant FET. After that it stays on until the batteries discharge completely (after that you need to connect the charger to turn the chip back on).