the way i do it is getting that finger under there the same way i'd do it when slapping. i find it easier to do when not trying to keep my thumb anchored on the B string (or pickup), but since i switch between 'anchored thumb' and 'floating thumb' often while playing, its not a problem for me.
Yeah, you do have to work on your timing (there are plenty of places i dont do it because it'll throw me off or i wont catch it in time). Also, string action makes a significant difference. Also, your TOP string (G on 4 or 5, C on 6 for regular tunings) is ALWAYS easiest to do this on, but if you cant easily fret a note on that string that fits, you can always mute the string and u'd get a toneless snap, which has a similar effect. I even do this trick with slapping. sometimes i cant slip my finger inbetween the strings fast enough on the D string so i just mute pop the G and keep it moving.
