It's caused by extra friction between the cue ball and the object ball. If one or both of them are unusually dirty and sticky they don't slide off each other the usual way. The cue ball will briefly grip or rub the object ball, shoving it a little in the direction the cue ball is already moving. Hence the ball undercuts.
In extreme cases a rolling cue ball will even climb up the face of the other ball, leave the table briefly, then plunk back down. This is the cause of the loud clack or thud sounds you sometimes get on a bad skid, it's the cue ball coming back down and hitting the table.
If you google Dr Dave billiards skid you can see a video of it in his site.
Posted from Azbilliards.com App for Android
Pretty sure that a skid or slide is due to NO friction not extra friction. You slide on ice right?? Thats because there is almost no friction between you and the ice surface. Same go's for pool balls and cloth.
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