During one game, the announcer said something about the Clock being stopped to move the chains. I guess that means that the clock would start up again as soon as the chains are in position.
that, of course, would mean that the length of the clock being stopped would change Depending on how far the team got past the first down line. If they barely made the first down, the clock wouldn’t be stopped as long as if they made a long pass.
I don’t know what the XFL’s rule on this is, but the description sounds very much like the NCAA rules.
In the NCAA, the clock stops after any offensive first down (it doesn’t in the NFL). It is restarted on the Referee’s ready-for-play signal. Any of the more commonly known reasons for stopping the clock (out-of-bounds, incomplete pass, penalty, time-out, etc.) would take precedence, though, and would result in the clock starting at the snap.
The language is pretty obtuse, but you can read about it in the NCAA Football Rules. This stuff is covered starting on page FR-59, which is in Rule 3, Section 2 (“Playing Time and Intermissions”).
The rulebook doesn’t address why they do this (that I could find), but the explanation I’ve often heard is “to move the chains.” BobT is our resident rules expert - I’m sure he could expand on this or correct me if I’m got something wrong. 
College football stops the clock to move the chains to make endings more exciting. Think about it, your team is down by 2 points, you’re on your own 35 yard line, there are less than 20 sec left on the clock, the QB chucks a pass 55 yards downfield to your star reciever in triple coverage. He catches the ball! In the NFL, it would all be for nothing. In NCAA, you can run up there, spike the ball, and set up for a field goal.
PS - Clemson, did make the field goal.
I was really hoping the xfl was going to be good. But its sure not worth suffering through a hangover to watch.