There are two “arcs” in the swing which must be understood if one is going to swing optimally, as they are not as you see in video or pictures – they are an optical illusion, actually.
The first arc is the vertical arc which, as I have combined two pictures that show the blurred arc of the club head before and post-impact, to show you what is meant by “descending,” “level” and “ascending” impacts.
As you can see, there is an arc here, and you cannot create a longer “flat spot” at the bottom of this arc – is impossible.
This also illustrates why a stable head is necessary to swing optimally – if the head moves left or right, that arc will shift, and then you won’t be where you wanted to be, if you were set up in the proper place to begin with.
The second arc is viewed from above, and shows another theoretical arc horizontally to the ground and again will illustrate why you can’t create a “flat spot” in your swing, and why, if you shift left or right during the swing, you will also shift the arc, and lose consistency.
And now we get to the reason these are “theoretical” arcs – you will never swing a golf club to create either arc in a regular swing – the vertical arc is that of a Ferris Wheel, and we don’t swing that way.
The horizontal arc is that of a Merry-Go-Round, and more of a baseball swing type action, and we don’t swing that way either (unless we are Ernie Els hitting out of the gorse at the British Open).
So, the two arcs combined – that is the true swing arc, and it looks like this:
The true swing arc is neither vertical nor horizontal, but rather inclined as you can see, following the club head in the swing gif. above.
I call the swing arc an “Inclined Ferris Wheel.”
And this is why, if you are going to swing a golf club with optimal athleticism and mechanical-correctness, you have to build a stance and have the mechanical action necessary to make your swing arc consistent and stable – meaning it’s the same arc every time you make a standard swing.