Lasso Tool Gone Retarded

[Dae]

I'm sure I'm using it wrong. I'm using the polygonal lasso to select part of an edge on my object. Sometimes it will select that part, letting me move it or delete it, but other times it selects that part, and deletes the rest of the object (aka. crops the part I've selected). This is really annoying, and its done it about 15 times in a row.

Can anyone help?


Thanks in advance :)