I am pretty sure in that picture that the punch is being withdrawn. The jacket is already split/creased at the desired length. Wiggling the split Portion of the case will cause it to break free from the case.
Here’s a set of pictures and instructions on using the dies.
http://www.corbins.com/et-2-s.htm
I made a pinch trim die for 7mm jackets and it works very well. You will need to heat treat the base punch at the least, since it has the sharp edge. I heat treated both punches. I used a steeper angle on the punch and my brass splits into about 6 petals. Most stay attached to the case until they are wiggled and then break free. If you were doing a lot of brass, I would think that you could tumble the cases to break the petals also.