[r0b1n]

I also sometimes do this (although not with tomatoes). But my knife for this is very sharp and hook-shaped (I think the english word is "turning knife"). Only thing you have to pay attention to: when cutting "against your thumb", never press your thumb into the knife, otherwise things will get bloody. The technique that works best imho is to e.g. hold a carrot in one hand, knife in the other, thumb fixing the carrot and knife edge cutting in a rotating motion around the carrot's axis beside your thumb.