From a conversation of the Lubavitcher Rebbe at the break-fast meal at the conclusion of Yom Kippur:[1]
“Rabbi Chaim of Brisk was customary to be lenient concerning questions regarding fasting on Yom Kippur.”
He once remarked:
“It is not, G-d forbid, that I am lenient concerning fasting, it is that I am strict regarding matters of life and death.”
[1] HaMelech BeMisibo, Vol. 1 Pg. 122