The free leg as it moves from its most recent position (as weight transfers onto the other foot), towards the point of collection/passing. After collection/passing point, it then extends towards the next position and becomes your 'leading leg'.
In salon & nuevo styles,strides are longer and sacadas are generally done close to the trailing foot...the idea is you need to occupy the position of the followers axis. Her axis was until a moment before, over her foot; so to put yourself there, you need to sacar or take away her foot.
This is a visual trick as well as physical reality.
Milonguero has shorter footsteps, so putting your foot in between her legs is approximately the same as putting it against her foot. That is part of the answer.
But the main thing is that you are still trying to occupy the space where her body just was. Sometimes that means taking away the foot,sometimes it means rotating around her axis, sometimes she rotates around your axis, sometimes you both rotate in balance.
In Milonguero, where your foot goes is about placing it under your shared axis, as a CONSEQUENCE of the intended movement, not the CAUSE of the movement.
In the end I don't think the concepts are so different as you would think between Salon/nuevo and ilonguero/Close-embrace.
Single Axis Turns and Colgadas:
Trying to occupy the SAME space as your partner is currently a popular field of discovery, with Single Axis Turns and Colgadas, or "Hangers", which are single axis turns where you lean back away from your partner.
Also, I've described a sacada as a switching of weight between lead & follow, whereas a displacement (such as off of the cross walk or the cadena) doesn't have the same "whoosh" quality that one gets with a sacada. Is this accurate?
I think many steps when done with confidence and groundedness have this whoosh quality. Sacadas, displacements and even normal steps all feel more powerful when driven deep into the earth.
A sacada does have this passing of energy into the follower, an exchange of momentum, as the leader takes over her space. This is especially true if there is pivoting or rotational movement occurring.