Back Button Focus

Was faffing around today after  watching a video about back button focus , So the above photo was taken using this technique . However when I went out into the field as they would say, things did not work out as I expected about 20/30 photos taken all of them were out of focus , don't know what I was doing wrong but will have to watch video  again to see what I was doing wrong. Is any fellow blipers use this technique ?