Jerry,
I have a similar mandolin to this, I have always struggled to get on with it,
the onion would move while in the guard, I found peppers were worse to slice, the veg were supposedly held in place by 3 prongs, peppers being hollow just would not stay put,
there were times it made me feel to try to slice things without using the guard,
which was plain daft,
maybe it was me, I don't know, but it now sits in the bottom of a kitchen draw not ever seeing the light of day,
maybe others have had a better experience than me with mandolins but they are not for me...
Mick