(Exodus 21:20) specifies that the master strikes his bondservant "with a rod." The Mekhilta asks: does this mean the master is liable regardless of what kind of rod he used? Even a thin reed that could not possibly kill?
The answer comes from (Numbers 35:18): "Or if with a wooden implement, whereby he can die, he strike him." This verse establishes that liability for killing requires using an instrument that has the genuine potential to cause death. A lightweight stick that could never kill anyone does not meet this threshold.
The Mekhilta derives the principle: the master is not liable for the death penalty unless he strikes the bondservant with something that has the realistic potential to kill. "With a rod" does not mean any rod. It means a rod of sufficient weight and force to be lethal.
This ruling served as a form of legal calibration. It prevented the death penalty from being applied to a master who disciplined a servant with a light switch or thin stick — implements that might cause pain but could not plausibly cause death. At the same time, it ensured that masters who used genuinely dangerous weapons against their servants faced the full consequences. The law distinguished between discipline and lethal violence based on the physical characteristics of the instrument used, not merely on whether the servant eventually died.