Falsely accusing an opponent or potential critic of the exact thing that you are saying or doing before they get a chance to call you out on what you are saying or doing.
Thus, bystanders and third parties become confused as to what is real and what isn't, and you stand a chance of getting away with what you are saying or doing.
He told a lie and then immediately said "but the fake news media willlie and disagree with me". Now the press can't call him on his lie without looking like liars themselves and neutral bystanders feel like they have to take a middle-ground. That's some shady liejacking right there.
Falsely accusing an opponent of the thing that you are doing before they can call you out on it. This sews confusion in the minds of bystanders and third parties, helping the liejacker get away with what they are saying or doing.
Donald Trump told a lie and then immediately accused the press of fake news. Now when they call him out on his bs, they look like they're lying themselves. That's some shady liejacking right there.