Old Navy's Kung Fu Knock-Off

I saw this t-shirt at a local Old Navy store. It is a knock-off of the Bruce Lee/Kung Fu type of t-shirt. The top two character says "Kung Fu (or Gong Fu)", but I don't know what the bottom two meant.



The phrase 勢力 means "power; (ability to) influence", but what is 力勢?



Especially when 勢 is written in Simplified version of 势. Any Bruce Lee's fan would know that he was from Hong Kong, which used the Traditional version of Chinese.



功夫 = skill; art; kung fu; labor; effort