[vikramkr]

CRISPR systems are molecular "scissors" you can use to cut a particular piece of DNA right where you want to, which is useful when doing gene editing. Usually, you put the scissors inside the cell, and it just sort of goes and cuts on its own timeline, you can't really control it. Here, they've designed a pair of scissors that will go right to where you want to cut, but then just hang out there not doing anything until you give the signal. When you give the signal (activation by light), the scissors make the cut "on demand."

So before, if you want to study/control the processes that go on after that cut, you'd be a bit out of luck since you didn't know when the cut would happen to start experimenting, it would be a hard messy problem. Now, you can just trigger the cut to happen right when you're ready for it.