A battery may be loosely modeled as a bunch of capacitors interconnected by resistors of various values. For simplicity, assume two capacitors--#1 is attached directly to the load, and #2 is connected to #1 via high-value resistor.
When the load isn't drawing any current, a small amount of current will flow from whichever cap has the higher voltage into the one which has the lower voltage. This will cause the two caps to approach an equilibrium where there voltages are equal.
When the load does draw current, however, charge may flow from #1 to the load faster than it can flow from #2 to #1. If this happens, the voltage on #1 will fall below that of #2. The greater the difference in voltage, the more current will flow from #2 to #1, but the voltage on #1 may fall below the minimum operating voltage of the phone even while the voltage on #2 is substantially higher. Once the phone shuts down and stops drawing power, the flow out of #1 will no longer be faster than the flow into it from #2, and consequently it will start to be recharged from #2 until the batteries again reach equilibrium.
Note that in practice batteries are a lot more complicated to model than capacitors; among other things, the resistances that connect the various capacitances are not fixed but vary as the battery is charged and discharged. Nonetheless, the interconnected-capacitors model provides a simple intuitive picture of what's going on.
PS--Both batteries and caps may be thought of as containing some charge-storing material and some material to connect it together. The more interconnecting material a battery or capacitor contains, the lower the effective resistances that connect everything together. The interconnecting material doesn't hold any useful amount of charge, however, so low-ESR batteries and caps have to be physically larger than higher-ESR batteries and caps with the same capacity.