I haven't watched the video and am not familiar with her work, but I have experimented with both strategies in the context of mood management, which might be similar, and I have indeed found that both strategies can work in that context. I've also found that envisioning failure doesn't work this way when I end up obsessing over it; it only works when I can accept that OK, I might fail, this is what failure would look like, I can live with that.
My own tentative theory is that the important part is accepting, rather than avoiding thinking about, the potential consequences of failure. (Or, come to that, of success.) I have no data to support this, though.
Imagining a specific failure, bringing it to a near mode, can remove the fear of unknown. Without fear, it is easier to work. On the other hand, imagining a failure is priming oneself to fail.
Which influence is stronger?
Maybe the result depends on how exactly, and how long is one imagining things. Methods similar on surface could bring different results.
the video
I'm posting this because akrasia, procrastination and willpower are often discussed on LW. I haven't read the book, but for those that are interested "The Willpower Instinct" and "Maximum Willpower" are, from what I can tell, exactly the same books.