Several times, the verb כלה (finish/complete) comes on the completion of an action. And when it is written לא תכלה פאת שדך (you shall not wholly reap the corner of your field)⁶⁸⁶, it comes to teach about a portion from a portion, that it is enough to leave any amount in the corner.

If it had said לא תכלה שדך (you shall not wholly reap your field), it would mean that one has to leave a portion of the field for the poor and for the stranger. The word פאת teaches that one is not allowed to wholly reap the corner of his field, that one must leave a portion from the corner of his field, a portion from a portion⁶⁸⁷.

footnotes: ⁶⁸⁶ Leviticus 19:9. ⁶⁸⁷ Malbim, HaTorah v’HaMitzvah Kedoshim 15.