Keith,
If the "keyhole" you are talking about comes from this post
http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=20;t=002335 I think its in reference to an earthfill dam. During the construction of an earthfill dam, an essential element is a keyway or core trench. The keyway/core trench is an excavated trench cut through the center of the dam. This trench is then filled with an impervious, compacted material(clay) that prevents seepage. Seepage affects the integrity of the dam and may lead to catastrophic dam failure.
An illustration of a core trench, with an explanation, can be found on pages 37, 39, 44 and 45 of USDA AG Handbook 590. I cannot get a link to the handbook but doing a search should get you there.
And to finally answer your second question, an excavated pond is basically a hole in the ground to catch water and, in most cases, does not incorporate a keyway/core trench.
Hope this helps.