Yep - it's one of those "knack" things that best comes from experience - the reason Justin calls them "flick-offs" (where many others refer to them as "pull-offs") is to try to create the image that you have to flick (drag down?) off the string to get it to sound.
Strength in hammer-ons also comes with time and practice - one tip, this is the one time it is ok to lift your finger right up off the fretboard, you need a bit of space to generate the right power.
Whilst exercises are good, finding songs that contain techniques is also good - for flick offs the classic riff lesson Justin does on Black Night by Deep Purple is good fun and not too hard - the flick-offs are in the part between 36s & 46s in his demo. And sounds great on acoustic.