How do Corkus Ampifiers actually work?

I feel with ampifiers, generally yes I get better rolls, but chance of getting 100% dropped instead??

 

it boosts by 15% if you would have gotten a 0% roll, and it boosts by 0% if you get a perfect roll. It basically makes bad rolls better while not having a big effect on good rolls

 

The answer above but more general:
It boosts only the failed parts of the roll.
i.e.
15% amplifier
actual roll: 40%
failed part of roll: 100% - 40$ = 60%
corkian ampifier bonus: 15% of failed part of roll = 15% of 60% = 9%
actual roll with 15% amplifier: 40% + 9% = 49% (in case of a decimal I believe it would be rounded down)
Also, unless it changed since they were introduced, they do not effect negative stats. So i.e. a knucklebones with a corkian amplifier can still get maximum negative life steal


EDIT
for clarification about how wynntils % works and how it influences this (originally left out because the part below does not really matter and is kinda technical):
rolls go from 0.3 * base to 1.3 * base in increments of +0.01.
This means that there are 100 different rolls you can get, which you might assume means that the % wynntils gives you is the roll. This is wrong, however.
As you might've noticed, not every stat goes up to 100 or more. Meaning that some of the rolls between 0.3 and 1.3 have overlap for which stat they give. Wynntils % does not look at the roll for its percentage, but at the actual ID that came out of it. This is why you can get 100% rolls without three stars in front of them. (three stars means a roll of 1.3)
The corkian amplifier however does look at the roll. In the example above, with a roll of 40% I actually ment getting and id of the value 0.3 + 40 * 0.01 * base = 0.7 * base. After which is decided which identification is gotten. With the corkian amplifier you would get 0.3 + 49 * 0.01 * base = 0.79 * base. So the amplifier only had any noticeable effect if the roll of the ID is a different value at 0.7 * base than it is at 0.79 * base.

My advice: Corkian amplifiers barely matter. They change the rolls of bad IDs to less bad but still not good IDs and they barely change the rolls of good ids. Use T3s when rolling mythics. But other than that do whatever with them.

 

this is pretty much correct; it's worth noting that rolls go from 0.3 to 1.3 so it's not necessarily a "percent", and also that this roll value is multiplied by the base value of the stat in question and the result is rounded, so rolls do not necessarily align with, say, wynntils %s

if you really want to get into the weeds of this, you can look at this spreadsheet and/or copy it for yourself and play with the values: link