Because I know Spanish pronunciation rules well, I can hear when my production is off compared to the source I'm shadowing. This lets me correct my pronunciation, as intended by the shadowing exercise.
With French, I'm finding that if I shadow out loud, my voice "clobbers" the source and I just carry on with my noob pronunciation. But if I just "lip sync" to the source, I suddenly start hearing all the French phonemes that are non-native to English and can start forcing my mouth to form those sounds (albeit silently).
I suspect this effect will disappear once I internalize the French phonology.
Just wanted to comment on this, in case it helps other beginners.
