Question

the component shown circled in the image here:
designed for locking a dimension by preventing motion of the spindle
provides a breaking away at a consistent gram force insuring repeatability
provides smooth uniform pressure independent of feel
permits measurements in narrow slots and tight places

Explanation:

The circled component on a micrometer - like device is typically a thimble lock. Its main function is to lock a dimension by preventing the motion of the spindle, ensuring the measured value remains unchanged.

Answer:

Designed for locking a dimension by preventing motion of the spindle