The best explanation of the concept has been given by John E. Thayer
in the help file to his game Computer Dots:
Dots is a complex version of the game played by school
kids over at least the last half-century. It is difficult to
pin down just when the game originated, but it remains a
popular lunch break pastime in many schools across the country.
When the game is played on paper, rows of dots are drawn
on a blank sheet to form a grid from ten to twenty dots square.
The first player draws a line between any two dots, then the
second player does the same. Play continues in turn as each
tries to draw lines that will keep the other from completing all
sides of a box. If a player succeeds in completing a box, he
marks it with his initials to claim it and then takes another
turn. When all of the dots have been used and no more lines
can be drawn play ends. Each player then counts the number of
boxes claimed. The player with the highest score wins.
Among the computer implementations below, Linx is a bit different,
because players try to complete triangles, not squares, something that
wouldn't work when playing on paper.