In this regard it's kind of like a fancy analog switch. The Curie is still sourcing and sinking current to the pins, but the voltage levels are clamped at 3.3V on the Curie's side of the chip.
It's not really a level 'shifter', and it's not a buffer. The board uses three Texas Instruments LSF0108 level translators to accommodate all of the I/O lines (including the analog inputs).
An Exploring the I/O Performance of the Arduino 101 webpage by Dan Watson has a number of relevant comments:
