The trick is to get your both hands in sync. Weather you do this with vinyl of the CDJ is no difference. This trick is more about cross-fader technique than deck-treck.
Basic principle : Open the fader for the duration of half of your sample, then close it. Backscratch the track while opening the crossfader for the duration of half of the sample.
Sync your left hand with your right hand... this can only be done by long practicing... It can take up to a month before you can perform a scratch... And even then you will make mistakes.
Learning how to scratch is a very time consuming thing and will take a year or more. But when you do master these techniques it's a big deal of fun to show-off
