We have had a reply from a source in U.S.
This is what we have found out...
"Over/Under is a term used to refer to lane conditions. Most times, the bowler is refering to the inside part of the lane (from around 2nd or 3rd arrow on the right to the 2nd or 3rd arrow on the left) is extremely heavy oil, and the outside part of the lane is extremely dry. When you keep the ball in the heavy oil, the ball will under react. When you get the ball out into the dry it over reacts. The best solution is to drill a bowling ball for this type of condition. With the proper layout, you will be able to play this shot rather well.
I find that pearlized balls are the worst on these type of lane condition. A ball with a dull finish will grab the lane slower and more smoothly."
I hope this is a help to all those who did not know.