That's a magical device alright... But for the truly pedantic...
It can only relieve straight-fluted cutters! For spiral (OK, helical) flutes you're into the weird world of geared relieving attachments which work by moving the tool rather than the workpiece - for them as is unaware of such....
Usually the spindle speed's reduced by an add-on gearbox sat on the ways in front of the headstock, driven by the spindle and with a matching spindle nose on the output side (often 6 or 8 to 1 reduction, call it 1/n);
A geartrain (with pick-off gears) separate from the QCGB / change-gears rotates (via universal joints and a telescopic shaft) a cam which moves the form-cutting tool into the workpiece, Firm Spring Pressure returns the tool to the rest position;
The topslide (compound) is attached to a slide which is under the control of the cam.
By varying the gearing driving the cam it's possible to get straight relief of any (practical) number of flutes by choosing integer ratios of cam to (output) spindle speed, if the ratios are non-integer then each relief starts at a slightly (or even very) different position around the workpiece, resulting in a helical relief - and a lot of calculations!
As the spindle's rotating the workpiece at constant speed it's possible to use the lathe's QCGB to set the pitch (and handedness) of the cutter teeth for making taps etc. (but set at n x desired tpi pitch - the 1/n spindle reduction, remember! For n = 8 cutting a 16 tpi tap would need a setting of 128 tpi on the QCGB - this may explain why toolroom lathes often go to what appear to be daftly-fine thread pitches?).
The Eureka etc. instead use a follower as in the Lessig video to establish the thread pitch, which works just fine but needs a different follower and thread prototype for every pitch... Some of the finer instrument lathes came with thread followers and sets of prototype threads as very pricey accessories!
The lathes best suited for these attachments have "double cross-slides" that put the lower slide under control of the cam (or taper attachment when not relieving) and allow the upper slide to be used for in-feed; better yet some of the official manufacturer's attachments have a third slide between cross-slide and topslide so they can cut helically-relieved, tapered cutters and taps! Too clever for me... I've not used one, but have long ago seen 'em in action a few times, attached to older Hendey, Pratt & Whitney and Holbrook toolroom lathes making special taps etc.
I suspect it'd be possible to home-brew a relieving attachment (assuming you have access to the quadrant gears to pick up the drive or can pick up from the spindle-speed reducing box), but a lot of work for someone much braver than me! A lathe with a taper attachment could be fitted with a cam/ follower assembly in place of the taper guide, perhaps? Of course, in these days of (relatively) inexpensive CNC there's probably an easy way to do it in software
![:)](https://madmodder.net/Smileys/default/side2side.gif)
Sir John would probably know.
Dave H. (the other one, rambling again)