Semantic hijacking consists in repeatedly using a term/symbol with a different (or even opposite) meaning to its actual one.
On the short term, this tactic is used to promote or slander certain target people/ideas by arbitrarily labeling them with words/symbols with a strongly positive or negative connotation: since most people do not reason on concepts but only on their name/symbols, they most likely won't notice the trick and will be pushed to love or hate the target thing without ever checking whether it actually fits the attached label or not.
On the long term, the repeated use of this tactic can lead to a complete inversion of the original meaning of a word and, together with it, of the whole beliefs and behavior of their supporters or opposers.
In case some progressive (i.e.: empowering or emancipating) ideal becomes too widespread to be directly countered, if that ideal can be identified by a name or a symbol, then it can be "polluted" by introducing extraneous concepts into its definition (or by expunging integral parts of it), and this can go as far as to entirely invert the original meaning; for the reason explained above, the majority of supporters will unconsciously accept the new meaning in order not to "betray" it. In this way is it possible for propagandists to push regressive agendas by hiding them inside widely upheld progressive names/symbols, making them harmless of even useful for the establishment.
The same tactic can be used in the opposite way, that is: to introduce progressive meanings into widely reviled names/symbols, in order to either prevent people from supporting them, or forcing them to support both at the same time; the ultimate result is exactly the same as the one described above.
The trick is to never definitely remove the original meaning, but to always keep a subtle level of ambiguity between the two opposite meanings; this ensures the tactic will never exhaust its manipulatory power (for example, by letting all people eventually re-adjust their thoughts to the new definition) and will always be useful to manipulate people in multiple ways at once: people who are first exposed to the "old" (progressive) definition will embrace the name/symbol and by doing so, they will unconsciously be manipulated into embracing and promoting its new opposite (regressive) meaning; specularly, people exposed to the "new" (regressive) definition will reject it, and by doing so, will also reject the progressive ideals it originally represented, with the additional result of promoting division and infighting between the two groups even though their actual ideals are very similar.
The combined use of these tactics on a vast array of terms over a prolonged period of time can result in a complete subversion of vast portions of language, with obvious disastrous consequences for any kind of rational debate towards any kind of truth or social progress.