The amazing "alien disintegration" effect--how was it done?

Can someone tell me how they did that amazing "alien disintegration" effect? It was certainly ahead of its time, especially for a 1960s TV show. On other TV shows of this era (such as Star Trek), when someone got zapped by a phaser or blasted by an alien death ray; it went into freeze frame and you could see how they optically printed the disintegration. However in this show when one of the aliens is hit, they disintegrate while still moving in the frame.

I this series back in 1967, and I've always wondered how they did that.