The phrase "long diagonals" refers to lines that go from one corner of a shape to the opposite corner, and are longer than the other lines in the shape. Full definition
You don't always need a midfielder who will always play long diagonal balls. (justarsenal.com)
He also tries one long diagonal pass that is unsuccessful, but also shows his versatility to get forward and attempt three crosses. (spursfanatic.com)
Make 3 inch long diagonal cuts on either sides or alternating sides about 1 1/2 inches apart using a sharp knife or scissors. (ambrosiasoulfulcooking.com)