Place the cubed chuck roast into the insert of a pressure cooker.
Whisk together the ingredients for the sauce in a measuring cup or bowl then pour over the beef.
Close the lid of the pressure cooker then set to manual setting then high pressure and 35 minute cook time.
In the meantime, cook your pasta.
Once pressure cooker is done cooking, quick release the pressure.
Remove the beef and place on a large cutting board and roughly shred it with a fork. Set aside.
Press the sauté button on the pressure cooker (if you don't have that option, you can pour the sauce into a pot and bring it to the stove) then once the sauce is simmering, whisk together the arrowroot powder and water then pour it into the pressure cooker. It should start thickening immediately. If it doesn't, you can make more of the arrowroot powder/water mixture.
Add the beef back in and toss to coat with the sauce.
Alternatively, you can add the pasta in too to toss or you can spoon the ragu over top of the pasta.