http://www.eijkhout.net/lead_follow/turn_foot_position.html
7.16 On Foot Positions For Turns And Spins
In her workshops, Beth Emerson recommends committing your weight immediately each time you step on a foot rather than having your weight split between feet for a moment, which is what we tend to do when walking. This makes the follower much more ready to follow instantly.
In one of her spins workshops, Beth Emerson showed how first and fourth foot positions were unstable but third and fifth foot positions were stable. She showed how the spin “prep” automatically puts the follower in third or fifth position, which makes her more stable going into a spin. She also talked about how people were usually taught to lead right on the beat, which for a spin would put the women in an unstable foot position to start the spin with. And that leads generally need to come right before the beat so the follower is in the correct position when she commits her weight to that foot on the beat. But then Beth showed the followers how they could put themselves in a more stable foot position before starting a spin even if the leader put them in first position. This takes some practice to learn to do at full speed.