A circle holds the sum of the digits along its arrow. So the circle is at least the smallest the arrow can add up to and at most the largest, given the candidates its cells still have.
It works the other way too: each cell on the arrow is at most the circle minus the least the rest of the arrow can add up to. A circle of 4 over three cells leaves each of them only 1 or 2.
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.
The arrow sets its circle
Follow the lit arrow from its circle to the arrowhead, and add up what its cells can hold.
A circle caps its arrow
The circle of the lit arrow is placed. The cells on the arrow must add up to it, each one together with the others.
Where it fits
EasyArrow Sum is an Easy technique: Easy Arrow puzzles are the first to need it.
The Arrow overview explains the rule and the levels.
Try it on an Easy Arrow puzzle. Next Step points it out when it is the move.