Basically, there's a brief lag in time from when you lead, and when the follower responds to the lead. If you lead and step at the same time, you'll be ahead of her, (maybe even kicking or stepping on her). I've found that there are some followers who don't like it when I kick them.
... the lead carries out a first barrida (sweep), answered by a barrida from the lady, and completed by another barrida from the lead. I understand how barridas number 1 and 3 are led, but what about barrida number 2?
My instructor said to lead the lady to take a step, like usual, but your foot precedes hers. Another thing you can do is "push" the lady's foot, and then reverse direction and it looks like she is "pushing" your foot (but it is not really pushing, more leading the step). I was able to lead this with a newbie once, I was happy about that.
Tango Distance brought up a very good point. The follower has to have excellent timing, especially if you want to barrida during the back-to-side step of a giro. If the follower releases the leg early after getting to her new axis (i.e. rushes her steps), then all bets are off.
A lot of times, it happens when she's not on the right foot to do what you ask, like if you lead her to her left while she's standing on her left foot (unless you want to hold her up). Leading an ocho but asking her to step before she's finished her pivot, etc. Sometimes your mis-timed lead could result in her taking a step instead of pivoting, or moving to the side when you wanted her to change weight in place, or do a forward ocho instead of an ocho cortada.
If you want to learn a dance from Youtube or the internet, I suggest you try a different dance - one that is built on a syllabus and judged from the outside, like ballroom. Tango is led from the torso - the lead connot be discerned easily by watching. I believe this is part of the Argentine culture - protecting your talent, your moves, your art, from those who would copy, is common in Argentine musicians, painters, writers - etc. DiSarli always tried to put his body beween the audience and his fingers, so no one knew how he was playing the piano. Music was collected after a practice, so musicians wouldn't steal the music. That is also why it is better to take lessons from a woman teacher - she has actually felt what a good lead does.
In my last lesson, we started to learn the barrida. I did not find myself terribly good. So, I watched the howcast video on the barrida, and noticed that their approach was slightly different than what I had done during my lesson. I still need to test it out, but it seemed a much better approach to me.
During today's lesson, I had the opportunity to try out Ana & Diego's approach. Well, it went very smoothly. I doubt I would be able to place that barrida in a real dance setting, but it was fun nevertheless.
From what [very little] I've seen of tango competitions (on YouTube, inevitably), they all dance exactly the same stuff in the same way, and then offer their syllabus-based nonsense to the judgment of their betters. This sounds just like 'ballroom', except that there isn't, and never was, such a dance as 'ballroom'.
The tragedy, for tango, is that last year's competitors (at least the ones that made the final rounds) are this year's 'Internationally renowned' visiting teachers/maestros, filling up their suitcases with foreign exchange, before going home to plan their next trip. All over the world, gullible dance student lap it up: and then wonder why the choreography they've paid so much for (and don't forget all those power walks: projecting the body powerfully forward, step after step) are completely useless if they ever go to a grown-up's milonga, where real people dance socially.