The digipot is vaguely similar to how a R2R ladder DAC works. There are two resistor values used: R and 2R. The "interesting part" of the circuit has bits x 2R resistors in parallel. One end of each resistor (let's call it input) is connected directly to the digital bit from the PWM sample.

Then there is the series bits x R resistors. The output of each 2R resistor is connected in-between the two resistors that straddle its bit position. So:

    Out
    |
    R
    +-2R-bit0 
    R
    +-2R-bit1
    ... 
    R
    +-2R-bitN

And that's it (apart from pull-up/down resistors)! Due to the way that resistors in series and parallel work, each input step in the ladder provides twice the voltage of the previous if powered: it's base 2 enshrined in a physical object.

These are relatively uncommon because they are highly sensitive to the resistor (and trace) tolerance, but this circuit is my Euler's identity of electronics: it very literally bridges the digital and analog worlds.