Can you begin the game with a tensioned rope/string?

[SenorZ]

R34
Non-electrical sources of energy used by the ROBOT, (i.e., stored at the start of a MATCH), shall come only from the following sources:

compressed air stored in the pneumatic system that is legal per R79 and R80,
a change in the altitude of the ROBOT center of gravity, and
storage achieved by deformation of ROBOT parts.

I did a quick check of the Q&A (https://frc-qa.usfirst.org/Questions.php) and couldn't find anything expressly prohibiting what you mention. But It was a QUICK look. Generally, if it is considered unsafe it won't be allowed. So don't make something that seems unsafe... or better yet, submit a Question to the Q&A.