Seems fairly simple to me. An adventuring day is defined in terms of XP (as a proxy for the number and danger of the challenges encountered) instead of time.
So, if the "adventuring day" for a party of 1st-level PCs is 1000 XP, a DM who wants to enable his PCs to complete the adventure before taking a long rest would put no more than 1000 XP worth of monsters, traps and other challenges in the adventure.
If the DM starts by designing the adventure instead, the total number of XP would give an indication of how many long rests the PCs will likely need to take before completing the adventure. For example, the PCs will need to take one long rest in the middle of an adventure with 2000 XP.
It's basically replacing an encounter XP budget with an adventuring day XP budget.
However, this does not shed any light on how the DM can encourage the PCs to press on if they decide to use all their daily abilities early in the adventuring day before earning their XP "quota" for the day (the "15-minute workday" problem), apart from the obvious solution of having only a single encounter that uses up the entire XP budget for the adventuring day.
And I'm sure that people who don't like the system will uncharitably misconstrue it as one in which the PCs stop adventuring and rest once they have hit their XP "quota" for the adventuring day regardless of the state of their daily resources at that point.