Mainly variable cover drivers that pick different jobs every week. Example, if a VC driver picked a Mon-Fri night job one week, and either worked through, or into the Sat. 1-5AM period, he/she would no longer be able to pick a Sun-Thu job, because the driver will only be off for one of those periods,
He/she could still pick a Sun-Thurs job, they just wouldn't reset. Then they would have to keep going back over the previous 7 days to see how many hours they had available. The IVIS actually can do it for you, just go into the DOT log and scroll down to the "summary" page. This tells you how many hour worked the last 7 days, the last 6 days, and how many hours available that day.
In the above example going form a Tues-Sat job to a Sun-Thurs, if they maxed out their hours they couldn't work Sun or Mon, if they only worked 40hrs Tues-Sat they would have 20hrs available Sunday and Monday, then on Tues the 8 hours they worked the previous Tues would fall off. On Wed another 8 would fall off, etc. If they worked 50 hrs Tues-Sat, they would have 10 available Sunday, and could not work Monday. Or depending on what time they finished Sat if they did not have hours available Sunday, if they had the 34hrs and two 1-5 am hrs off they would reset Sunday, this would all depend on the start time Monday.
If you violate the DOT rules and get a letter from the company, (or fine from the DOT) you learn how to figure your hour out real well. dahik.