This is from the September edition.
"Dozukuri and Monomi"
胸尻出でて 顔は反りけり (大和)
Dou no fusu ite ni (amata) no nan zoaru
mune-shiri idete kao wa sori keri
"An ite with a protruding torso has many difficulties; with the chest or buttocks sticking out and the face bent."(Oowa)
As described above, when our head is bent backwards it can be said that our "monomi is tilted upwards slightly". When our monomi is tilted upwards, our eyes and ears are not in line with each other, our sense of balance in our semicircular canal is not normal.
This reminds him a time during the 2009 Berlin World Championships in the 200m dash finals watching Usain Bolt's running form. The 200 meant he had to run on a curve first. But when you see his body, it was not bent at all and his head was straight in line with the ground as he ran. He broke the world record at 19.19secs.
The deepness of the monomi is shown in the diagram on pg 116 of the (Japanese) Kyohon Vol. 1, but we should not force it to be too deep or else the left shoulder starts to form a triangle shape. It should be moderate.
The deepness of our monomi will also effect where the arrow is pointing towards the target, so one needs to be careful about this.