A weld on thumb is one that has its own pin. The thumb is welded to the dipper stick slightly above the center of the lowest pin on the stick. These thumbs can have slightly greater rotation than a non link pin based thumb, but are weaker and more prone to twisting due its hinge point being only as wide as the dipper stick versus how a pin based thumb is connected on the outside of the bucket pin. Also the thumb "slides" on the bucket cutting edge during rotation due to it having a different rotation point than the bucket. So, a thumb that just reaches the bucket edge when fully rotated, will be long by several inches when it is in a more curled up position.