You are correct. "Up one's sleeve" originates from magic. A magician often has "tricks up his sleeves" ... *LITERALLY* ... for instance, cards up his sleeves ;). The phrase can just be used to describe a hidden talent / secret, particularly a hidden talent that is likely to help whatever person has it. "I don't know if Pepsi can really last much longer. Coca Cola is just dominating the market". "Yes, but James [the managing director of Pepsi, for instance] always has another trick up his sleeve". It means that the managing director always has an unexpected [hidden] idea ... and he'll make sure that Pepsi continues to survive. Hope this helps and good luck!

up (one's) sleeve : Hidden but ready to be used: I still have a few tricks up my sleeve. The words usually used with these are "Tricks" ... And it means that the person still has some hidden tactics / ideas in his possession and that he may use it when needed.