:-? Hardy a U-turn. When a sailing boat is sailing with a direction that is at 180 degrees to the wind, it 'tacks' so as to zig-zag in a way that is ultimately against the wind. At the end of each of these zigs or zags it 'goes about'.
The word 'tack' has given rise to a metaphor that you may have met: to 'take a different tack' is to change the direction of an argument, but with the same ultimate objective.
b
(I imagine Rover knows this, and certainly 5jj does - as I believe he has sailed more than I have.)