Place the chicken breasts in the Instant Pot insert in a single layer if possible.
Pour the zesty Italian dressing and the 1/2 cup water over the chicken.
Sprinkle the chili powder, ground cumin, minced garlic, and the ranch seasoning packet evenly over the chicken.
Secure the lid and set the valve to sealing. Cook on high pressure for 12 minutes for fresh chicken or 17 minutes for frozen chicken.
When the cook time ends, perform a quick release according to your pressure cooker's instructions.
Open the lid, remove the chicken to a cutting board or bowl, shred with two forks or tongs, then return the shredded chicken to the pot and stir to coat with the cooking juices.
Serve immediately.