From the stickied turnip guide:
"Shield attacking turnip:
While it is hard to do and hard to position, it is possible to do heavy damage to the opponents shield by throwing a turnip 'into' their shield, It should be roughly positioned so that Peach's extended hand is in the centre of their shield. Normal turnips will linger around the shield after the first hit, sometimes bouncing twice, or sometimes not allowing the opponent to jump from shield, A powerful turnip like stitch, dot and also mr saturn can cause a lot of damage to the shield if it double hits."