A GFCI will detect the amount of current passing from hot to neutral. and whether there is any irregularity, in which case it trips the circuit thereby cutting of the current. An irregularity could be that part of the current is flowing elsewhere, say through a person. A standard GFCI can read a disparity as little as 4 milliamps, and react as quickly as 25msec.
A sight line is the linear landscape right before your eyes. In other words, it’s what you see when you stand at the entrance, at the table, or at the island and look forward. With good planning, your eyes will rest on something pleasant and balanced.