To trigger this you need to make Khabko meet Reznikov without any security present.
Firstly, shoot the flag winch to set Khabko into his leaving routine. Then shoot the release for the main exit to the prison (in the bottom floor of the building nearest it, just through the door window) - this will bring Reznikov down to check on the riots.
Meanwhile, there are two guards stationed outside the main building (with the star on it). Lure them away and hide them in the snow banks one by one. Then lure away and silently take out the security following Khabko and Reznikov (one each).
Reznikov will be checking on the riots and giving instructions, shoot each gate opening mechanism for the pen he's currently looking at (he'll look at 3 in turn), this will cause him to return to the main building, where Khabko is waiting.
Khabko will then assault Reznikov. If there are guards present he will start choking Reznikov then stop when they raise their guns. Without the guards however, Khabko will kill Reznikov and then begin to escape, at which point the challenge will pop.
(Credit to Pacifistycuffs)