I think as a general rule, there is a lot to be said for the idea of taking the time to get the basics down and understand the fundamentals before moving on. In the context of a confused student looking to catch up, however, this rule goes from generally good to absolutely essential. The kids who are likely to seek out this kind of assistance and, more to the point, those who really need it are almost without exception afraid of the material. This anxiety is usually one of the main obstacles to mastering their lessons
One of the great advantages of watching a math video is that it doesn't watch you back. The fear of looking dumb, of being judged, is perhaps the main reason struggling students are reluctant to ask for help. Being able to listen to an explanation or see an example in a safe and private place can be remarkably beneficial but those benefits go away quickly if that video makes the viewer feel stupid or confused.
The first requirement of any math video (or math lesson, for that matter) is that it be correct, not necessarily in the picky, get-the-jargon-right sense, but correct in the sense that the basic ideas are right and you end up where you're supposed to. A close second is that the video make the student comfortable with the material. If I'm explaining a problem to a student or group of students, I can adjust my presentation based on the reactions I'm getting. I can slow down, make an explanation more detailed, offer words of reassurance, even stop what I'm doing and switch over to a simpler problem. The video instructor can do none of these things.
That said, under the right circumstances, a video that walks a student through a problem and hits just the right level of difficulty can do a lot to build both skills and comfort level.
Here's a clip that does a pretty good job, particularly with the reassurance part.