A knockout tie that goes beyond ninety minutes stops testing the same qualities it was testing before. Extra time favours depth, and a shootout barely relates to the football that preceded it.
The extra half-hour is a fitness contest
By the end of normal time, both squads have used most of their substitutions and the players remaining have covered a full match.
The additional period is played at lower intensity with longer gaps between actions, and space opens because pressing structures can no longer be maintained.
Teams that pressed hardest earlier often suffer most, having spent the reserves that extra time then demands.
The extra substitution changes the calculation
Competitions permit a further change once extra time begins, which gives a coach one fresh player at the moment fresh players matter most.
Deep squads can introduce someone capable of running past tired defenders, while thin ones are choosing between fringe options.
Coaches sometimes hold a substitution back through the closing stages of normal time for precisely this reason, accepting a cost now for an advantage later.
Why the shootout is a separate competition
A penalty shootout is a sequence of set-piece attempts taken with no defensive pressure, no time constraint and no relationship to the pattern of the match.
The skills involved are technique under pressure and goalkeeping anticipation, neither of which the preceding two hours has been measuring.
This is why the better team over a tie loses shootouts at a rate that feels wrong and is entirely consistent with how the mechanism works.
Preparation does affect it
Goalkeepers study opposition takers, and clubs practise shootouts under conditions designed to reproduce fatigue and crowd noise rather than in isolation.
Order of takers is planned in advance, since the pressure on each attempt varies enormously depending on the state of the sequence.
These preparations shift the odds modestly. They do not turn the shootout into a reliable measure of which side played better.
What it means for previewing a knockout tie
Ties between closely matched teams reach extra time often enough that bench quality deserves weight alongside the strength of the starting elevens.
Away trips, midweek congestion and recent minutes all feed into who still has legs at the end of a long evening.
Beyond that point the tie is decided by a mechanism nobody has found a way to forecast, which is a large part of its appeal.