So I recently purchased a Petzl Stop to satiate my curious 10 year old, who's showing an interest in Rappelling. (The plan was to start with a figure-8, but that's another story... LOL)
After playing with it myself, and re-configuring my cowstails and ascenders to the "European" way, I must say, holy-moly that's a nice setup. Performing a changeover as the previously posted British woman in the video was just great. I imagine it wouldn't take much time at all to perfect a changeover onto a bobbin, with my eyes closed, spinning on the rope, etc. (In a waterfall, now that might take longer to get used to... LOL)
I then tried this same setup & changeover with a rack, instead of a bobbin. Attaching the rack below my chest ascender, and then disconnecting the chest ascender. The first attempt was a failure, and resulted in me getting stuck... As others mentioend, there's so much slack in the rope above the rack, it's [nearly?] impossible to get enough slack out for a changeover. The climbing ascender was way too far up, and I couldn't get the slack out in order to weigh the rack, and unload the climbing ascender. I had to hang on the climbing ascender (single point) whilst I unthreaded the rack and then reattached the chest ascender, then downclimb to the ground.
I'm going to try this one more time with the rack - attaching it below the chest ascender, to see if I can position both ascenders better for a down-climb onto the rack. Probably need to downclimb farther down towards the rack, before removing the chest ascender. Otherwise, it'll be going back to the "American" way for racks, where we use the QAS/3rd ascender to aid with changeovers.