Help me Understand Flash Exposure

Hi,

I just did a little experiment with on-camera external flash in a completely dark room so I could see the effects of different settings on the flash.

I noticed a few things:

1. There's an aperture setting for the flash itself, at least in Manual mode - what does it do? I took shots with different settings and noticed no difference (though I only looked on the back of the camera and zoomed).

2. When I changed the power of the flash (again, Manual mode) from 1/128 all the way to 1/2, there is absolutely no change in the histogram. The camera itself is also on Manual mode so it didn't compensate or anything. I thought I'd get a way overexposed image at 1/2, but apparently not so.

3. In Manual mode in the flash, I see no option to enable High Speed Sync (which is available in TTL mode) and the camera, as expected, won't let me set a shutter speed above 1/250. Is this normal to not have High Speed Sync in Manual mode?

If it matters, I use a Nissin Di866.

Thank you.