For the Leaders: Sacada is one of a class of steps where your chest is doing one thing while your legs and center of gravity are doing another. In pretty much every sacada, the leader is saying, "go this way" while he is not going that way, so he can go through the follower's leg. The need to multi task in this way is something that is very typical of many steps in tango.
When David is trying to learn steps that have more than one thing going on at once, he takes his time and finds the coordination that connects the two things. Nancy suggests isolating that one step which embodies the particular difficult coordination and practicing it until you feel natural doing it.
(From Wikipedia): Known as La Negra (literally: The Black One), Mercedes Sosa was an Argentine singer who was popular throughout South America and many countries outside the continent. With her roots in Argentine folk music, Sosa became one of the preeminent exponents of nueva canción. She was best known as the "voice of the voiceless ones".
