Why was Jota goal not offside - and does law need changing?

Had James Tarkowski not intercepted the ball, Luis Diaz would have been called offside.

But because the Everton defender did, his defensive instincts kicking in, Diaz was onside.

This moment in Wednesday's Merseyside derby led to Liverpool's winning goal, scored by Diogo Jota.

Everton were not happy. So what are the offside laws and are they fair?

What happened?

Image caption, Ryan Gravenberch tried to pass to Diaz, who was standing in an offside position. Diaz made no attempt to move towards the ball or challenge Tarkowski...Image caption, ...but as the ball came in, Tarkowski slid in to prevent the ball going through to Diaz. Liverpool then seized possession - and seconds later Diaz teed up…