The complement technique is the simplest basic technique.

Presence of a true sign

When there is a true  sign in a grid, we can complete the boxes in the corresponding row and column with false signs.

By the very definition of the integram, there can only be one true sign per row and per column. Therefore, all other boxes are false.

Presence of n-1 false signs

With n = number of boxes per row. When a row is full of false signs except for one box, it necessarily contains a true sign.

Still because of the definition of the integram. A row contains only one true sign, the rest are false.