The concept Annabelle used was simple, you twist the end to lengthen of compress the brush. This is what I found each feature did for my lashes : Lengthen = spreads out the bristles, allowing you to coat each lash giving length and volume; Compress = tightens the bristles allowing you to wiggle in and flare out the lashes for extra definition.
The way I like to use the mascara is use the lengthened brush first, which gives me length and deposits enough product for crazy volume, then I switch to the compressed version to help flare out the lashes and remove product build up. I actually find the tighter the bristles, the more natural and voluminous my lashes are. I usually follow the same steps, using two different mascaras, when I really want a dramatic lash and have the time for it. Having both these features in one product is very convenient.
The formula is similar to Marcelle Xtension Plus, but I like it a bit better. It's not a very wet mascara, but it can set fast so work quickly. I can easily get two coats on and that's enough; a third would be too dramatic for me. I know, right? It's that effective. It lasted well during the day, with no smudging or flakes and it came off easily at night with an oil based remover.
Here is a before and after gif.
Annabelle's Expandable Mascara should be out on shelves by now, I got mine at my local Pharmaprix.