no, a thumb, when you dally the horn and the thumb gets caught under the rope, and steer hits the end of the rope. A lot of them are missing a thumb, which eliminates the problem forever. Fortunately, it’s the weak side thumb however.
Usually the thumb, but other fingers have been smashed and lost. Not sure what you mean by “the weak side thumb”. If you are right-handed, you throw the loop with your right hand, jerk the slack with the right hand and dally with the right hand, so its a digit on your right hand that is at risk. The left hand holds the reins and the coils of the rope.