That hooks to the intake (air filter arm) right next to the throttle dashpot vac. hose.

I'm not real clear on why that bypass is there- it appears that if it senses a vacuum in the intake arm(or maybe a differential between before/after the throttle plate?) it opens to allow some air to bypass the butterfly, raising the idle a little bit. Note that its big hose is pulling air from the EACV supply passage and routing it directly to the plenum. I just went out in the yard and looked at 9 different manifolds. Half have it, half don't.

Fwiw, my b16a doesn't have one. It's the early version with the 2 O2 sensors.