Basically it's a low-passed reese (two saw waves that are detuned from each other to make them rhytmically beat against each other) that's been distorted (commonly with wave-shaping distortion) either before or after the lowpass filter. A few points about making one:
Remember to detune the oscillators equally far up and down so that the note you're playing stays in pitch.
You could always automate the second oscillator's volume in and out to make a bit of a smoother sound due to the phase cancellation (the rhytmic beating of the oscillators cancelling eachother out) losing it's effect when the LFO dips. If you do this and your synth allows you to key-track the LFO's rate knob, you can still maintain the change in the beating of the oscillators that's essential to reese sounds.
And if you'd want to dissect that further, one could argue that detuning evenly apart and then automating the volume of the other osc would lead to the sound not being in pitch, and be entirely correct. However, discerning frequencies gets harder in the sub frequencies, and of course you could tune the non-vol-automated osc slightly less away from the desired pitch.
