A captain whose side leads by a large margin after the first innings may ask the opposition to bat again immediately. The option is used far less than it once was, and the reasons are practical.
The rule offers time, not runs
Enforcing the follow-on saves a captain the hours their own side would spend batting a second time, converting that time into overs available to bowl the opposition out.
In an era of slower scoring, that saving was decisive, because a side batting again might use two sessions to set a target and leave too little time to bowl.
Faster modern scoring rates have eroded the advantage. A side can now build a substantial lead quickly and still leave ample time to take ten wickets.
Bowling workload is the main deterrent
A bowling attack that has just taken ten wickets is tired, and asking it to bowl again without a break risks injury to fast bowlers in particular.
Fast bowling loads accumulate across a series, and a captain who exhausts an attack in the second Test may be without it for the fourth.
Declining the follow-on gives bowlers a rest measured in hours rather than minutes, at the cost of surrendering some of the time advantage.
Fourth-innings pitches are not always worse
The traditional logic assumed the surface deteriorates, so batting last is hardest. On many modern pitches the deterioration is modest and a fourth-innings chase is viable.
Drop-in surfaces and improved preparation have reduced the extremes, which weakens the argument for making the opposition bat immediately on a still-good pitch.
Where the surface does break up sharply, the follow-on remains attractive, and captains in spin-friendly conditions still enforce it regularly.
Psychology cuts both ways
Batting again with a large lead removes any prospect of defeat and demoralises an opposition facing a target that grows with every over.
Enforcing the follow-on, by contrast, offers the trailing side a route back into the match, and a long second innings can shift the momentum entirely.
Captains have become more risk-averse partly because the rare reversals are remembered far longer than the routine victories that enforcing produces.
The margin required is fixed by format
The lead needed to enforce the follow-on is set by the laws and scales with match length, so a shortened match requires a smaller lead.
Weather interruptions can therefore change the option's availability partway through a game, since a match reduced in length lowers the threshold.
Captains track that threshold during a first innings, because a declaration timed slightly differently can preserve or destroy the choice entirely.
