Maxim's Smart Power Selector technology allows the system to use external power while the battery is in a deeply discharged state.
An on-chip MOSFET between the system load output (SYS) and the battery (BAT) serves multiple functions during charge and discharge operations. During charging, this Smart Power Selector switch makes the best use of limited USB or adapter power, utilizing input power not needed by the system to charge the battery. It also lets the battery serve as a storage buffer, supplying load peaks that may momentarily exceed the input current limit. During discharge, the switch provides a low-loss path from the battery to the system.
See diagram attached.