A Sequence in 12 Years A Slave shows Patsey, a plantation worker, being tied to a pole and whipped.
The shot is horrifying, and shows her flesh being torn and rendered open by the whip as it makes contact, cleaving chunks of flesh away.
Obviously they didn't actually whip Lupita Nyong'o, and from this interview we can determine that no actual contact was made with her body during the scene:
It’s all about the crack of the whip. You hear it. And you feel it. I felt the wind of it every single time. I didn’t need much more. That was one technical thing, and definitely it took some finessing because obviously I can’t see what’s happening. I can’t see the whip. I can only react to the sound and the wind of it, yeah? So it was hard, but that day was as real as it could have possibly been for me, because in preparing for it, all I could do was be present.
It's unlikely the shots were achieved by CGI, unless it's literally the best CGI ever used in cinema and Steve McQueen is incredibly humble about what he's achieved (those of you who have seen the movie will no doubt vouch for this, it's incredible).
But I can't understand how they could have been achieved practically without physically touching the actress, which she has stated didn't happen.
Another interviewer was, like me, utterly incredulous as to how this scene was shot and asked Nyong'o herself, but she only replied:
It feels, as the viewer, like it would've taken longer to achieve -- not only because it's one continuous shot, but because it's so emotionally heavy. Were those practical effects or CGI effects employed?
I don't feel like telling! [Laughs]
Does anyone with any practical experience of S/FX understand how they could have filmed this? Does anyone actually know how this was achieved?