Riding, running or swimming close behind another athlete to cut air or water resistance — on a bike, up to roughly 30% energy saved.
On a bike, air resistance is by far the dominant force above roughly 25 km/h, and drafting cuts it drastically — 20 to 40 percent depending on gap and group size. Running, the effect is smaller but noticeable into a headwind; in open-water swimming, sitting on someone's hip also saves measurably. Two things to know: first, drafting on the bike leg of age-group triathlons is generally forbidden and carries time penalties — the permitted gaps are in each event's rules. Second, it's a skill: in a wheel you're riding blind and see potholes and braking too late. Ride offset and with a bigger gap before you slot into groups.