Anyway Mike, back to topic.
In my Aikido as I've said the extending of the finger is to direct Ki.
Sometimes when I see some more disciplined Ki direction is needed I devote the lesson to this one fact. It's all to do with 'extending' Ki for real rather than imagining or thinking you are.
In these exercises all leading has to be done with the finger or more precisely through the finger.
When leading straight down for example it means two things.
1) Straight down means straight down. Not a slight angle etc.
2) To a specific location ie: Australia. (through to Australia) rather than to the floor. The location may be a specific point on the wall or through that specific point.
There are reasons for this and when done precisely the effect is totally different.
For all the machanics people out there I would say this: Just like in electricity flowing from one node to another. You have to have two stable points for the power to flow.
The idea of the fingers/hand directing the Ki is an old one which was originally important in the scheme of things, but the point is that this facet of Ki is a secondary by-product of the so-called Internal Strength skills being discussed in various forums here and in other places on the internet. Sun Lutang, the famous Xingyi and Bagua expert, changed the way the upper hand is held in San Ti so that the palm is more or less pointing forward; the palm had previously been down with the fingers pointing forward so that the qi would go in a forward direction. The Old Guard protested the change, but Sun simply indicated that they didn't actually understand the fingers-forward relationship to qi. I could explain the situation, but once again it is a topic that to do justice would take several pages.... let me just say that if someone is using good ki/qi skills the whole finger-pointing thing is sort of a non-issue. You'd probably be far better off to explore some of the physical aspects of ki (jin/kokyu).... they're productive studies.