How exactly do I "Initiate a power attack", to do this special knockdown move i got from Manny?

Hold down the attack button.

If you're talking about the NCR takedown move, walk backward while holding the attack button. Is this a spoiler thread? Can't decide.

Hold down the attack button and tap the left anolouge stick (im on 360 sorry, might be the same for ps3 nout sure), in a certain direction while at the same time letting go of the attack button (you got to be unamred) after charging it up, it should then execute a speacil attack if you learned it, for example Ranger Andy in Novac taught me the ranger take down, so I hold the attack button, flick the anolouge stick downwards to initatiate the ranger takedown then let go off the attack button sending my enemy flat on his ass :toughninja:

Ah sweet thanks guys.

Im on PC, but I get the scenario.