For a ko, a player can virtual-force something if he can force the something while exceptionally he may always make a ko-capture on this ko immediately after the opponent's ko-capture on this ko unless that ko-capture was preceded by the player's ko-capture on a different ko.
Without restricting and unrestricting extras, the sentence's core is: "A player can virtual-force something if he can force the something."
A restriction is added: "For a ko,". This means that the core refers to an arbitrarily chosen but then particular ko. Since the core does not itself refer to "ko", the core itself is not restricted for itself yet. Only the extension to the core refers to "ko" and is therefore restricted.
The extension to the sentence's core is: "while exceptionally he may always make a ko-capture on this ko immediately after the opponent's ko-capture on this ko unless that ko-capture was preceded by the player's ko-capture on a different ko."
How is that extension restricted by the restriction "For a ko,"? The extension uses "on this ko", again "on this ko", and "on a different ko". The restriction means that both phrases "on this ko" refer to the same ko and that that ko is arbitrarily chosen but then particular during application of the definition text. I.e., first we choose a ko, then we apply the definition text to it, and during the entire application that ko's intersections are referred to. The same two intersection during the entire application. It is that ko's intersections we are interested in because the phrase "on this ko" is referring to the ko's intersections. - It does not matter if the ko temporarily cedes to be a ko during application because the ko was initially given, from then on we know its two intersections, and then for application of the extension text with respect to its reference to the restriction "For a ko," we are interested in only those intersections. - The phrase "on a different ko" refers to a ko existing on two intersections that differ from the "For a ko,"'s ko two intersections. This is so because of the very meaning of the word "different".
Here "while" means during each moment of the player's attempt to force.
The definition exists in a context (of other definitions) where it awaits the input of given rules. Those are being applied during application of the definition. Such given rules have to have or imply the Basic Ko Rule. Now the "exceptionally" overrides the given rules. More specifically it does so to override the Basic Ko Rule by specifying exceptions.
"he" is "the player".
"the player" is not "the opponent". "the player" and "the opponent" are the exactly two different persons in alternation.
The exception's main clause is: "he may always make a ko-capture on this ko immediately after the opponent's ko-capture on this ko". IOW, whenever the opponent has just made a ko-capture on this ko, then immediately afterwards the player may recapture this ko.
This exception is an unrestriction of the core because it provides the player with extra rights.
The exception itself is in turn restricted by a subclause though: "unless that ko-capture was preceded by the player's ko-capture on a different ko". This is when the "always" of the exception's main clause does not apply.
What do exception and subclause exception of the exception mean in effect? There are two cases:
a) The player has made a ko-capture on a different ko and then the opponent has made a ko-capture on the particular ko. Then the player's next move may not be a ko-capture on the particular ko.
b) The player has made a move that has not been a ko-capture and then the opponent has made a ko-capture on the particular ko. Then the player's next move may be a ko-capture on the particular ko.