The follow-on lets a captain make the trailing side bat again immediately. It was designed to prevent time-wasting, and its modern use has drifted from that purpose.
The problem it was written to solve
A team far behind on first innings could once occupy the remaining time simply by batting slowly, securing a draw without ever competing for a win.
Allowing the leading captain to send them straight back in removed that refuge, because the trailing side had to bat out a much longer period.
The rule therefore protects the result rather than the leading team, ensuring that a substantial advantage can actually be converted into a win.
How the margin works
The follow-on becomes available when the leading side is ahead by a defined margin at the end of the first innings, and the threshold varies with match length.
Shorter matches use smaller margins, on the reasoning that less remaining time makes a large deficit harder to overturn in any case.
Enforcement is entirely optional, and the decision belongs to the captain of the side that leads rather than to the umpires.
Why captains increasingly decline
Enforcing the follow-on means the bowling attack goes straight back out, and a tired attack in hot conditions loses pace and accuracy quickly.
Batting again instead gives bowlers a rest, extends the lead beyond any realistic chase and allows the captain to declare at a moment of his own choosing.
The cost is time, and a captain who bats too long can find the weather or a stubborn partnership removing the win he was protecting.
The condition of the pitch enters the decision
On a surface deteriorating quickly, bowling last is unattractive because the fourth innings offers the greatest assistance to whoever is bowling then.
Enforcing the follow-on effectively swaps the innings order, sending the leading side out to bat last on the worst version of the pitch.
Captains weigh that against the risk of giving the trailing side a full day of recovery, and the balance usually depends on how much time remains.
Why it survives despite falling out of use
The rule is rarely enforced at the highest level, which raises the question of whether it still does the job it was written for.
Its presence continues to matter even when unused, because the threat of it discourages a trailing side from batting purely for time in the first innings.
A rule that changes behaviour without being applied is doing its work, and that is the strongest argument for leaving it in place.
