Placeholder in sortable

Placeholder in sortable


In layman terms i need to be able to see where i am placing an object
in sortable, like on igoogle and bbc homepage.
Here's my CSS:
.sortableHelper {
background-color: yellow;
opacity: 0.6;
z-index:1;
}
And here's my JS:
$("#columnLeft").sortable({
connectWith: ["#columnRight"],
placeholder : 'sortableHelper'
}
I thought this would show a yellow block where the object is going to
be dropped, but it doesn't.
I am lost to quite what the placeholder option is meant to do and i
can't find any example of what it does.
Any help would be greatly appreciated.