Interesting, but anybody got any 'Why'?

I have some intuition: as the water turns to ice it's clear, but once the ice is formed, any additional cooling causes expansion, which causes internal cracks due to the space constraints from the container or the surrounding ice? Obviously frost on the surface isn't going the uniform, so his second example isn't great.

Water normally has some dissolved air in it, which is expelled and trapped when it's frozen leading to the white spots in the ice. To make clear ice you need to avoid the dissolved air being trapped in the ice.

One way to do this is to boil the water before freezing, which reduces the amount of dissolved gases.

What I think is happening here is that freezing only part of the water, and relatively slowly, lets the dissolved gases remain in the liquid part of the water.

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Impurities settle over time, so freezing slowly from the top allows time for the impurities to fall to the bottom of the container.

This is a shitty rip off of http://videosift.com/video/How-To-Make-Clear-Ice-Cocktail-Chemistry