Any failed roll and a 1 on the Pro Die triggers a Major Failure. The closer the d20 is to 1, the worse the effect, to give the DM/Player a guide. A roll of 1 on the d20 is always a Crit, same as 5e RAW, but if you ALSO roll a 1 on the Pro Die, something really bad has happened.
If you roll a 1 on a hit, it’s a normal +1. The 1 on the Pro Die is only bad if you fail. When you roll without the Pro Die on skills and attempts you don’t have proficiency in, there are no Major Success/Failures. They are only tied into the Pro Die, so Crits and Partials are the only things that can be triggered without the Pro Die.
As players level they can up the Pro Die to a d6 and eventually a d8 for a skill or save. This allows you to do some really cool stuff as you level, but there’s still a chance of that 1 showing up, but it’s less likely.
Save rolls use the Pro Die for the 2 stats you assigned them to, but they never trigger Major Fails/Success on saves.