It calibrates (+/-25 points throw 10 games) from your non ranked MMR. There is no start from ground Zero. If you had success in ranked games more then in non ranked, then your MMR should be higher then your new calibrated "international" MMR. This is only useful for people that played Ranked right after they had level 13 and were new to Dota so they got stuck in very low MMR. Later on they got much better in Non ranked games. There for their new International MMR will be based on their non-ranked MMR that should be higher then the low Ranked MMR they got stuck in.
It is also a bit of a joke since it's not an actual reset of MMR. But since the majority of your games are Normal it is a good representation of your current true MMR values.
For the record my First MMR was calibrated at 3,5k and I had 54% win rate back then.
Now I have 49,X% Win rate and am calibrated at 3,2XX K, that is still about 100 MMR more then my current Ranked MMR. But 500 MMR points are sort of your floating space usually that you go up and down till you get better to go up. In my case I got 100 points out of it and the knowledge Normal games are generally more normal Dota then Ranked Games. I feel like in Ranked games there is so much tension going on and people tend to be even more selfish then they are in Normal, because everyone thinks his skills are much above his current MMR and it is his job alone to win this.
I still think this is a good option for you to reset your Ranked MMR back to your Normal MMR, if you got very bad luck with your Ranked games. If you are paling mainly Ranked and your Ranked is higher then you need to grind more skills no other way to rise more then you have.