Cross booking is called back to back booking. Here's how it is done:

You need to travel from JFK to SFO on Tuesday and return on Friday. Fare for this itinerary is $1,350. Insted of buying this ticket, you do the following: buy a ticket from JFK to SFO on the day you want to travel but with a bogus return date that involved a Saturday night stay.Price-$390. Then you buy a second ticket originating in SFO, going to JFK.Price-$390. The outbound portion of that ticket is the day you really want to return to JFK. You create another bogus return date on that ticket that also includes a Saturday night stay. This allows you to buy two highly restricted tickets that could be significantly cheaper than the one JFK-SFO you originally wanted. You end up travelling exactly when you wanted and you throw away the two unused portions unless it is a route you travel regularly, then you use the second portion of each ticket and beat the system again.