This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The defect known as a windshake is so-called on account of the belief that it is caused by the racking and wrenching to which the growing tree is subjected by high winds. It is also claimed that it is produced by the expansion of the sapwood which causes a separation of the annual rings from each other, thus leaving a hollow space in the body of the trunk and following around between two of the annual rings. Fig. 5 shows the appearance of a windshake on the cross section of a log, and this appearance has given rise to the term cupshake which is sometimes used instead of windshake. The hollow space may extend for a considerable distance up the trunk of the tree. Windshakes are very frequently found in pine timber.
Fig. 4. Section of Log Showing Heartshake.
Fig. 5. Section of Log Showing Windshake.