Consider this sentence:
I don't know which kind of filtering they used that can't even be opened by a proxy!
or
I don't know which kind of filtering they used that even it can't be opened by a proxy!
What is the structure of such sentences? "Which kind of .... that ... even ...."
For another example consider:
I don't know which technology they used in this mobile set that even works under water.
Is it a correct sentence. If yes what is the structure of it? Is it a relative clause?
You ask:
What is the structure of such sentences? "Which kind of .... that ... even ...."
The pattern to which you're referring is, I suspect, a rhetorical statement whose tone is sarcastic, and the irony is expressed by the speaker's feigning ignorance.
I don't know what's so "advanced" about the camera, when it doesn't even have a low-light setting.
I don't know what's so "advanced" about a camera that doesn't even have a low-light setting.
Beats me what's so "advanced" about a camera that doesn't even have a low-light setting.
The speaker here portrays himself or herself as someone who lacks the knowledge or brains to say why the camera can be called "advanced"; his or her limited sense of how things ought to be would require an "advanced" camera to have a low-light setting. The speaker is at a loss to say why it's "advanced". Of course the actual meaning is "I know what features an 'advanced' camera should have, and this one is lacking in that regard".
Correct placement of a that clause:
Incorrect placement: I don't know which kind of filtering they used that can't even be opened by a proxy!
Correct placement: I don't know which kind of filtering that can't even be opened by a proxy they used!
Or
I don't which kind of filtering they used; it can't even be opened by a proxy
I don't know which kind of filtering they used, which can't even be opened by a proxy.
the that clause has to be attached to the antecedent (the noun or phrase) it is attached to. The book that was on the table was closed.
However, here, you might want two sentences, as your that clause is so long: I don't know which kind of filtering they used. It can't even be opened by a proxy.
THIS: a that has to immediately follow what it refers to. "the kind of filtering that can't be opened by proxy" is the phrase. Not: "the kind of filtering they used that can't be opened by a proxy".
Because then, this too would be acceptable: - the game they played that cannot be beat. - the movie we saw that was so good.
Those should be: - the game that cannot be beat that they played - the movie that was so good that we saw.
Some people in speech do tack on verbs after the noun to which the that refers, but it can lead to confusion and should not be used in writing.
Correct placement of that clause when that is a relative pronoun: I don't know which technology they used in this mobile set **that even works under water.
That can be used as a subordinate conjunction:
He knew that the card players were cheating. [notice the worlds knew that]
As a subordinate conjunction The word “that” can be used as a subordinate conjunction, that is, as a word that marks the beginning of a dependent clause.
As a relative pronoun The word “that” can be used as a relative pronoun; in other words, as a pronoun that begins a subordinate clause and is the subject of that clause. A relative pronoun not only marks the beginning of a dependent clause, as a subordinate conjunction does, but also, unlike a subordinate conjunction, acts as the subject of that dependent clause."
Therefore:
I don't know which technology they used in this mobile set **that even works under water**** is a correct sentence as **that marks the beginning of the clause and is its subject.
In my other answer I was guessing at what you were trying to say, since the meaning of the filters and proxies example wasn't apparent to me. I guessed wrong. But now that you have cleared this up with your comments, here's a second go at an answer.
You're trying to express the idea
This filter is strange or novel. It cannot be opened by a proxy, but most filters can be opened by a proxy.
Your first statement is therefore OK:
I don't know which kind of filtering they used that can't even be opened by a proxy!
but your second is unidiomatic and ungrammatical:
I don't know which kind of filtering they used that even it can't be opened by a proxy!
You're using even unidiomatically there, for one thing, placing emphasis on the filter when your meaning requires emphasis on a proxy's usual ability to open filters. But let's set that issue aside and focus on the ungrammatical.
The pattern of your desired sentence can be paraphrased as follows:
How can this be a normal or typical {x} when {y} is true?
or
I don't know what sort of {x} this is when {y} is true.
How can this be a normal filter when a proxy cannot open it?
If you use that instead of when to introduce your subordinate clause, it is ungrammatical to repeat a reference to the {x} in the relative clause as you do
...filtering...that even it...
The noun filtering is the antecedent of pronoun it in the relative clause. That is a no-no:
How can this be a normal or typical filter that it cannot be opened by a proxy? ungrammatical
But a reference to {x} of the main clause can be placed in the relative clause if you use when to introduce the relative clause:
How can this be a normal or typical filter when it cannot be opened by a proxy? grammatical
