Straight back leg lifts

In the midst of looking for videos I felt needed deleting from the favourites on YouTube I came across this video:

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(Skip to 3:22)

I remembered that the reason I favourited that vid was because of how straight her back leg lifts/split kicks were. I was wondering if there were any progressive exercises needed to achieve that, as I don't think back leg lifts alone would accomplish this.

To kick that high, you need to develop the flexibility of front splits. A good tool is doing back leg kicks with theraband.

Another is splits against a wall.

You need to really loosen up your hamstrings as well. I know this from every time I try to merely extend a scale/arabesque higher than horizontal.