TORIKAKE 

Proper TORIKAKE 

The space is needed between the nocking point and thumb (just the width of a shaft). In this way, you can avoid too much contact between the thumb and the nock. Also, the index finger properly supports the shaft in this way.   

Bad TORIKAKE

Too much contact between the thumb and the nock causes the problem: During Hikiwake (drawing process), the thumb might push the nock out of the strings. That cause serious problem: the shooting impact damages the bow, which might be broken. 

Following conservation law of energy in physics, the total energy is conserved in the shooting process. However, the problem is that all the energy is not turned into kinetic energy to fly an arrow, but some t of the energy lost in impact on a bow, depending on how to shoot. This impact damages the bow, which might cause the bow to be broken. If the string is too tough to be durable with this shock such as one for western archery, the impact is much stronger than hemp string and soft type synthetic strings such as Senbon-tsuru. Hemp string and soft type synthetic strings such as Senbon-tsuru will be cut with the impact, instead of damaging the bow.

Proper TENOUCHI placement

This way is Heki Ryu way, but the basics are the same regardless of the sects, especially from Dai-san.  

Place KOKOU (the part of the left hand between thumb and index finger) at the point 70% of the width of UCHI-TAKE from the right side (30% of the width of UCHI-TAKE from the left side). The hollow space is needed between the palm under the thumb and Nigiri. Also, place Tenmonsuji (the vertical groove of the palm of the hand, which appears when the palm is bent to grip something) at the corner of the left side of To- Dake. Then gently place the little finger, ring finger, and middle finger without pressure, which are aligned with the side of the bow.

Proper TENOUCHI movement

The little finger, ring finger, and middle finger are placed but not fully gripped, leaving a hollow space between TODAKE and these three fingers. The bow is firmly supported near the base of the palm and thumb, and from Dai-san, you can add a twist in the direction (the blue arrow indicated) 

Bad Tenouchi

Too tightly grip the Nigiri, index finger is sprained, etc.