Sails are boat's engine and they produce power in one of two ways. When the wind is coming from the side of the boat, it flows around both sides of the sail (like an airplane wing), creating lift which "puls" the boat forward. When the wind is coming form behind the boat, it "pushes" against the sail and simply shoves the boat forward.
One boat is clear astern of another when her hull and equipment in normal position are behind a line abeam from the aftermost point of the other boat’s hull and equipment in normal position. The other boat is clear ahead. They overlap when neither is clear astern. However, they also overlap when a boat between them overlaps both. These terms always apply to boats on the same tack. They apply …