OT: how does a laser tape measurer work?

My Bosch one uses a phase shift rather than time-of-flight, this chap did a tear-down and reverse engineering, in several parts

Yep. There are several types - with phase-shift devices typically limited to shorter distances than time-of-flight devices. Typically. Not Always!

And (also typically) much more accurate than a long metal tape, and easier to use than a cloth tape.

Peter Wieck Melrose Park, PA

Some professional laser devices used by surveyors and civil engineers do require calibration. As when two ends of a bridge meet in the middle, 1/8" (3.175 mm) does matter.

Peter Wieck Melrose Park, PA

Admittedly I skipped thru most of that, but I did not see what the function is of this close-coupled mechanical litteral gating .

Mine does give a mechanical 'clunk' when you press the measure button, he describes this as a curtain that opens/closes to measure/calibrate.

Sounds a similar function to your gate, but mine doesn't rattle if you waggle it.

There is not enough inertia in the mass of the gate itself, plastic about 5x5x1mm, to be noticable in normal handling, it is totally free to move though when unpowered. It seems what you/I sense is the gate being suddenly stopped at either end of its swing and the inertia of very fast movement is transfered to the body of the unit.

My guess for the swing gate is, there is an optimmum power setting for a given distance measured for accuracy, some middling setting initially and then up or down the power of the laser after initial measured distance.