Personally i'm thinking that hand seals are only used for setting the user's chakra in a certain mode. Once the hand seals are performed, the chakra is 'set' to that mode. for e.g. 2 hours after the seals are done, the jutsu can be executed without doing the seals again.

I think thats why sasuke didn't need to redo the hand seal when doing chidori against itachi in ep84

This is just my opinion/theory though.