Hammer ons and pull offs are a fretting hand trick. Hammer ons use percussive pressure from you finger hammering a string down on the fretboard, to sound a new note.... (no strumming hand involvment, other than to keep out of the way and prep for the next note in the phrase).

Pull offs, are accomplished by plucking a string, not with you strumming hand, but rather with your fretting hand once again. It all happens at the fret board. If you are playing a note and, instead of picking with you stumming hand to get the next note in a phrase, you quickly pull you fretting finger off sideways. If done right, you can sound the next note in your phrase. Again... no strumming hand involvement other than to be ready for the next note comming up.

Hammer ons and pull offs are strictly a fretting hand trick. It allows you the ability to pick notes faster than you would if you had to use your strumming hand to pick all the notes. Because you are changing which hand does picking in mid phrase, it takes a lot practice to do either of these moves smoothly ....

I didn't say that it was esay :-)