The resistor is mixing the output of the second last stage back into a point earlier in the chain, and that's what produces the resonance. Look closely and you'll see that the output of IC2a goes to the switch and from the switch through a 15k resistor to the input of the 3rd phase-shift stage (IC5b). The Phase100 has 10 phase shift stages, 4 that are fixed (IC5b, IC6a, IC3b, and IC2a), and 6 that are swept by the modulation/LFO circuit (IC6b through IC1a). So, of the 4 positions you can have wider sweep with and without resonance, and a narrower sweep with and without resonance. Both switches adjust how much resonance or feedback there is at the same time as adjusting the sweep width. The 4-position switch does two things at the same time, that are similar to what the Color switch on a Small Stone does.
The schematic for the original (I have no idea if Dunlop used the identical circuit beyond the status LED) can be found here:
