Ok, you have my "thumbs up" on this one.
My initial thought was that it looked a bit over complicated, but after giving it a second glance I think the complication serves some of the purposes that make for a good back stop. For anyone that doesn't have a back stop for their lathe, MAKE ONE NOW - of whatever sort works for you at the moment. You'll make more in the years to come as you find the need.
Some of the criteria that should be taken into consideration for a back stop:
- Very quick and easy to insert and adjust, without removing anything significant
- Positive locking positioning, both in the spindle bore and for the depth rod
- Balanced - no need for your lathe to shimmy across the floor when you crank it up to turn a small diameter
- Rigid - The end of the rod shouldn't "whip" at high speeds, or vibrate at any time
If you can build in some flexibility, that's great. But if you have a job that will work with something simple and single purpose, that's fine too.