Intent of move
The first square that a piece moves to signals its intent of move. This is especially important for rank (horizontal) moves of the queen and rooks.
For example, a queen on a6 moves to b6 signaling that it will be travelling along the 6th rank. It must end its move on the 6th rank unless it enters an empty field and decides to occupy the other empty square (in this case on the 7th rank).