The line of pennants five metres out from each wall. Swimming backstroke it is your only early-warning system: from the flags on, you count strokes to the wall.
Backstroke is the only stroke where you never see your target, and that produces the most expensive reflex in the pool: lifting your head to peek. It tips your body position instantly, your hips drop, and the wall still arrives as a surprise. The flags solve that without looking anywhere — from there it is exactly five metres. The real mistake isn't missing the flags, it's failing to count afterwards: work out once how many strokes you need from the flags to the wall; for most people, depending on height and stroke length, it's around three to five. From then on you touch the wall on your last counted stroke with an outstretched hand, under control, rather than with your head. What often gets missed: your number is personal and only valid for that pool at that pace — switch from 25 to 50 metres, or lengthen your stroke in a race, and you count again.