Using an ordinary sharpener kept upside down works best for me.
And this is why:
My beloved Bruynzeel Design Pencils are professional coloured pencils. But they are not the most expensive ones and not the very best quality. Don't get me wrong, they are good quality and I love colouring with them but the quality of wood casing and leads varies a little.
Some pencils have very good quality wood and others less.
The good quality pencils are easy to sharpen, it doesn't matter how you keep the sharpener.
But the lesser quality pencils need 'special treatment'.
When I keep my sharpener with the sharpening blade on top, I will have to push the pencil a little upwards to make contact to the sharpener. I found that this pushing up frequently is the reason why wood breaks off the pencil and points crumble or break.
When I keep the sharpener upside down I allow the pencil to rest on the inside of the sharpener without me pushing the pencil down.
Now I gently sharpen the pencil and gravity will bring the pencil into contact with the sharpening blade.
This way of sharpening has helped me to reduce broken points and cracked wood!
Also very important, I regularly replace the sharpening blade. A blunt blade will ruin your pencils.
You can also sharpen your blade but I prefer to replace it once in a while.