And his hand tightened so on the reins that some fraction of his reaction must have reached Shiloh.
His tightening hand had no impact on Shiloh as they never felt his reaction. 
mnli hypothesis: His tightening hand had no impact on Shiloh as they never felt his reaction.  premise: And his hand tightened so on the reins that some fraction of his reaction must have reached Shiloh.