Synonyms: elite troops, rapid deployment troops, special troops, storm troops.
(Believe it or not, it has nothing to do with crack cocaine. This term has been used since before jesus was born. Sun Tzu, the man who wrote "The Art of War" well over 2000 years ago used this term in his writings. Of course, back then, people who used the coca plant didn't call it crack.)
This term was simply based on the analogy that employing these troops could create "cracks" in an enemy's military structure or strategy.
"Do not follow a feigned retreat. Do not attack crack troops" -Sun Tzu "The Art of War" (the book on Armed Struggle)