https://www.carbonbrief.org/explainer-the-ratchet-mechanism-...
e.g. A nation will propose an INDC it thinks it can realistically achieve. It will be informed it's INDC is inadequate and that "more ambition is needed". So, they'll be forced to adopt an INDC that is not realistic. They'll fail to implement this, but they still won't be able to create a realistic INDC for their next five year period. Even after submitting an INDC they know they have no hope of implementing the country will likely be informed, once again, that "more ambition is needed".
Here's the thing. While the INDC's will steadily climb in their ambition, everything I've read indicates that there is no enforcement mechanism to see that they are implemented. In this sense it's Kyoto all over again. However, if the goals are too far out of reach, will we even try to reach them?
If you have information to the contrary, I'd love to see it.