I thought that notch would have been part of the latch mechanism to lock if anything but couldn't see how it worked. Now that I thought about it some more, maybe it's similar to a BP. Like you pull down & push up to make the triangular key lock in & out of the notches? I can't really tell if the lever can do that though.
Look at the first pic.....the screw that holds the top link on is off center to create an eccentric. The triangular block engages into the notch when flipped the opposite way to hold it open, not closed.
It looks like it is using an overcenter lock design, real similar to how many box and cabinet latches
The lower piece likely piviots where the red circle is. rotating it clockwise, brings the upper pivot (green arrow) past the center of the line from where the lever pivots on the right, and where it pivots just below the red circle, ending up approximately where the green circle is. The spring is then trying to pull the lower piece further clockwise, but is prevented by the part (where the red arrow is pointing) that mates into the cutout (yellow arrow)on the arm.
That's an ambitious modification. I think the RF-30 needs a spindle lock for sure; a spindle brake maybe not so much. It's a heck of a lot better than relying on my bad habit of grabbing a handful of belt while loosening the draw bar. Would make locking and unlocking keyless chucks easier, too. Thanks for sharing the idea!