Cup Shake  - Cracks or seams are separations of bark or wood.  The separations are circumferential or radial in wood and usually vertical in bark.  In wood, the circumferential separations or the weakened areas that are prone to separation are called ring shakes.  In wood of some living trees, the shakes are open.  In other cases, the weakened zones are called shake lines.  Cup shake is a ring shake that circles the central portion of a trunk.  Heart shake means wood separations in the center of the trunk.  The separations may be both circumferential and radial.  Radial shakes or ray shakes usually follow the sheets of ray parenchyma and the shakes usually start from a circumferential shake.  Many wood separations do not become apparent until after the tree is cut and the wood begins to dry.

Source: A New Tree Biology - by Shigo  For much more see source Chapter 36 for starters.

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