Clicking on this once while chaining will cause the Mac to "lock" its screen at this point in the chain. Clicking it again at a later point in the chain will cause the Mac to unlock its screen again. Changes that occur while the screen is locked will not show up until the screen is unlocked.
This is handy for situations in which you want more than one part of the screen to be hilited simultaneously. Start the chain, click the LockScreen button, then click each of the Hiliter buttons involved, then click the LockScreen button again, to produce such an effect. The effects of Flasher buttons and of Shimmerfields, Bulger fields, TypeWriter fields, and AutoScroll fields are suppressed while the screen is locked.