The official Scrabble rules contain the following:
Premium squares only count on the first turn they are used on. If a subsequent turn uses a letter that was previously placed on a premium square, the tile on that square is only worth its face value.
The only time a premium square is active for more than one word is if more than one word is created with a letter on a premium square on a single turn. Only use premium squares that are under tiles used in a given word when calculating point totals for each word created in a turn.
For example, if you create ‘jobs’ and ‘cakes’ by adding the ‘S’ tile to the end of the existing word ‘job’, and the ‘K’ tile is on a red square, you would count ‘jobs’ for its face value (13) but you would triple the value of ‘cakes’ (30) and your total for the turn would be 43 points.
I find this a bit confusing. Can it be explained further?