وَإِنْ كَانَ الْبِئْرُ مَعِينًا لَا يُنْزَحُ، وَجَبَ نَزْحُ مَا فِيهَا: أَخْرَجُوا مِقْدَارَ مَا فِيهَا مِنَ الْمَاءِ
Translation: If the well is fed by a spring and cannot be emptied, an amount equal to the water then present must be withdrawn.
Ma‘in describes a well replenished by continuous inflow. Since literally emptying the well may be impossible, the ruling is implemented by removing the estimated quantity present at the relevant point.