When the bounds are not enough, list every way the arrow can add up, using only the candidates in its cells. A candidate that is in none of the ways cannot be right.
Two cells on an arrow that share a row, column or box cannot hold the same digit, so some ways drop out. Cells that share none of them may repeat a digit, so 1 + 1 can count.
Worked examples
2 positions from real Arrow puzzles on this site, the familiar case first. Step through each one: the board lights what each step talks about, and the red lines show what rules what out.
Two cells that share a house
Two cells on the lit arrow are in the same row, column or box, so they cannot hold the same digit.
A digit can repeat
The two cells on the lit arrow share no row, column or box, so they may hold the same digit.
Where it fits
MediumArrow Combinations is a Medium technique: Medium Arrow puzzles are the first to need it.
The Arrow overview explains the rule and the levels.
Try it on a Medium Arrow puzzle. Next Step points it out when it is the move.