One of the gripes I've noticed here and on other forums from time to time with the Sony flash system (and the minolta flash system before it) is that you need to use the built-in flash to trigger other flash units wirelessly. The HVL-58 unit can also be used to trigger some flashes (if I recall correctly - haven't had the spare cash to get one yet).
I was mucking around taking taking some shots with the Twin Macro Flash and with the Ring Flash - doing comparisons etc, and setting up a shot for a poster on my baby's bedroom wall. The set up I had was a bit dark at the back (it's meant to be like a blue sky), and I wanted to add another light behind the subject, but there wasn't room for a continuous light. I had a small reflective table from my macro studio to provide a black relective surface, and a blue backdrop - but the backdrop was too dark, even with a pair of lamps on it:
I wanted to use a 5600 under the table to light the backdrop, but to trigger it wirelessly. I was thinking this was a problem, when I thought of the Hot Shoe Slave Flash Trigger for Sony / Konica Minolta flashes
I'd bought from Gadget Infinity last year.
I was putting the flash (lying on it's back) under a small table for macro stuff, and there was no line of sight between the camera/ring flash I had mounted and the flash unit or GI slave trigger, but it worked all the same:
It seems to work just about anywhere - any flash will trigger it, any time
Now, maybe I'm just very slow off the mark and everyone else worked this out ages ago, but I was pleased to find a workaround to allow me to use any hot show flash to trigger any remote flash!
Gadget infinity sell the slave triggers for US$15.95 (€ 11.76) on their website, or they may even be cheaper through their ebay store (that's where I got mine).