I would think pressure cooking chicken breasts in the instant pot for longer, and by that I mean 12 minutes or a little more, they would come out super tender. Am I wrong? Would the opposite happen? I may try it as an experiment.
Time in the pressure cooker will depend on the thickness of the chicken breasts. I'd say 12-15 min on high pressure should give you meat that can be shredded easily (I've cooked a few times following some variation of this recipe, which yields similar results).
Cooking any meat on high heat (or high pressure) for an extended period will both:
- denature more the proteins / collagen making the meat more tender;
- "squeeze" more water out of the meat, making it drier / chalkier in texture.
At 12-15 min range the chicken meat will dry out some, but let it rest in the cooking liquid for a few minutes before cutting and it should be able to re-absorb some of it (or not release more).