Sure, let me tell you this thing first 'Difficulty increases based on number of miners mining a particular coin'.

So, for lets consider KMD coin for an illustration :



For my configuration (8 x 1060G) WTM says I can earn 8.31 KMD and 0.00254 because the current difficulty has dropped 57.9% compared with 24h difficulty, but if difficulty reaches back to 24h value I can only mine 3.50 KMD or so, similarly if the difficulty increase may only get less coins

This thing only happens if the big farm holders switches to another coin (that's my assumption), so during that time if other miners try to mine they (may) get more yield with same electricity. The difficulty may not stay low for long time but its good to mine when it is low (24h low we should call).

Papampi's WTM switch also does the same thing but it doesn't look for difficulty drop, it looks for current high BTC revenue from the selected coins and switches to mine that coin.

Hope i explained it clearly, please do ask if you have any questions.