You might have also had the board pinch the blade and cause friction burns or blade stalling. In short, Reaction Wood is abnormal wood formed in a leaning tree. In softwood trees, the reaction wood forms on the lower side of the lean and is called compression wood .
Compression wood is often very dense, hard, and brittle.
In hardwood trees, reaction wood forms on the upper side of the lean and is called tension wood . Woolly surfaces and excessive longitudinal shrinkage are often symptoms of tension wood. Reaction woods should be avoided for a number of reasons. The dense hard wood is less likely to accept an even stain when compared to other parts of your project. The reaction wood is also more prone to failure under load and will crack and split more easily when nailed or screwed.