In the 19th chapter of Tanya the Old Rebbe continues to explain about the hidden love. It says in the Torah that one's soul is like a candle, meaning: just like a fire always wants to go up and disconnect from the candle, even though it'll go out, so to the G-DLY soul naturally wants to leave the body, even though the body will be nothing. This is not understood, because it's higher than understanding. and so it's by anything holy, however, the other side which is the opposite of holiness always wants more, and it wants to exist. So are sinners which didn't get to be threatened for their lives if they don't serve an idol, since they sin and the animalistic soul rules them and covers over all their good this love is called "hidden love"

Now the Rebbe asks, if the hidden love is covered how does it affect the person when his life is threatened to rebel against G-d? The REBBE answers: This is a level which is higher than understanding which is called "Chachmah", Chachmah itself isn't cover, rather its power which spreads to the whole body is covered. So the level of Chachmah isn't covered but it's "sleeping" and when a person's life is threatened to serve an idol (This is called Kidush Hashem) it "wakes up" and its powers are revealed, and the sinner gives up his life without understanding why. All of this is because of the hidden love which we have from Abraham, Isaac and Jacob.