This quotation is from Dynamic Judo Throwing Techniques by Kazuzo Kudo (9th dan), p. 27
Seizing your opponent's practice suit—whatever part of his suit you grab, remember to do so using your little finger and ring finger together. If you are taking hold of his jacket, thrust your little and ring fingers in, and then catch the material and pull. At the same time add your index finger, and relax your thumb. Let your thumb lightly rest on the cloth of his jacket, because if you tense your thumb you will spoil the movements of your feet.
The author suggests that tensing the thumb hinders foot movements. How is this? What foot movement is more difficult when the thumb is used to grip?
It is also curious that this author suggests three fingers be used: pinky, ring, and index. This is contrary to the modern advice I hear to grip using the lower three fingers: pinky, ring, and middle.