What is a "whiffle tree"?

For the uninitiated:

whif·fle·tree

n. Northeastern U.S.
Pronunciation: 'hwi-f&l-tree'
The pivoted horizontal crossbar to which the harness traces of a draft animal are attached and which is in turn attached to a vehicle or an implement. Also called singletree, swingletree; also called regionally whippletree.
[Variant of whippletree.]
Regional Note: Whiffletree, a term primarily used in the northeast United States, is derived from the older term whippletree, which is used in the Upper Northern states farther to the west. The fact that whiffletree, the newer term, is used in the Northeast, the older dialect area, illustrates the process of linguistic change. Even as the older word whippletree was spreading westward into a new dialect area, it was evolving into something differentwhiffletreein the area where it originated, as if the older dialect area were somehow trying to keep a step ahead.

Shew! Probably much more than you wanted to know about the etymology, but I like to be comprehensive ;-)