My take:
'His body was tensed as a strong leaf spring drawn to the breaking strain'.
It's a simile.
The tenor/target is the tension of his body.
The vehicle/source is the tension of a strong leaf spring was drown to the breaking strain.
leaf spring: A composite spring, used especially in automotive suspensions, consisting of several layers of flexible metallic strips joined to act as a single unit. (The American Heritage Dict)
breaking strain: the point when the spring breaks due to excessive tension.