Repetition changes the information
In the regular season a defence meets an opponent a handful of times, usually with a day's preparation and a tired roster. In a series it meets the same five players, the same first action out of a timeout and the same counter to that action, every other night for two weeks. Everything that was disguised by variety becomes visible, and a coaching staff with film and time will eventually name the two or three things an offence genuinely relies on.
Once those things are named, they can be taken away. Not perfectly, and not without cost, but enough that the offence has to reach for its second and third options — and second and third options are, by definition, the actions a team has practised least. This is the mechanism behind the familiar observation that playoff scoring falls. It is not that players stop trying. It is that they are being asked to do the thing they are worse at.
Rotations shrink and hiding places disappear
A long regular season forces coaches to play deep into a roster, and a deep roster always contains a player a defence can attack. In a series that player simply does not play, or plays six minutes with heavy help around him. The whole floor gets harder to exploit, because the weakest link has been removed rather than protected.
The same logic runs the other way. An offence that beat the league by hunting mismatches finds that the mismatch it wants is on the bench in the fourth quarter, and that the switch it counted on has been replaced by a scheme designed specifically for it. What looked like a repeatable advantage was often just an advantage against average opposition preparing for eighty other opponents at the same time.
Adjustments are concessions, not solutions
The word adjustment suggests a fix. It almost never is. A defence that decides to trap the ball earlier has decided to accept four-on-three behind the trap. A defence that stops helping into the paint has decided to accept shots at the rim. Watching a series properly means watching which concession each staff has chosen and whether the opponent is capable of punishing it repeatedly.
That is also the fairest way to judge a losing team. A series lost because the opponent made the concession shot at an unusual rate is different from a series lost because the offence had no answer at all, and the difference matters when the same two teams meet again. Arena's postseason guide covers the format itself; this piece is about what the format does to the basketball played inside it.