loading saved state (altirra) on real atari

A while ago (in a thread about Syncalc) I got a bootable art with on this disk something that I would call a 'saved state loader' ... it loaded a saved state from Altirra into atari memory, and then continued the running on the real thing. Just like loading a freezer dump.

Is this a feature? Does someone knows how this works? I could not find a feature to create a bootable ATR to create this.

Thanks
M.