"For whoever does work upon it, that soul shall be cut off" — the Mekhilta specifies that this refers to "a complete work." The full prohibition applies only when a person completes an entire forbidden labor, not when they perform a fraction of one.
The Mekhilta tests this with a practical example. If someone wrote one letter of the alphabet on Sabbath morning and another letter towards evening — is he liable? He has performed the forbidden labor of writing, but in two separate installments rather than one continuous act.
Similarly, if someone wove one strand of fabric in the morning and another towards evening — he has performed the forbidden labor of weaving, but not in a single continuous action.
In both cases, the person is not liable. The work must be completed as a single, unified act to trigger the punishment. Fragmentary performances of a labor — split across time — do not accumulate into a full violation.
The proof comes from the Torah's emphasis: "And you shall keep the Sabbath, for it is holy to you. Those who profane it shall be put to death. For whoever does work upon it, that soul shall be cut off." The phrase "does work" — singular, complete work — establishes that liability attaches only to a completed labor. Half a labor in the morning plus half in the evening equals zero violations in the eyes of the law. Wholeness, not accumulation, is the standard.