Send Backwards (legacy) is one of the options for handling errors. It is present only to provide backwards compatibility with robots created by Design Studio versions prior to 8.0 and should not be used in new robots.
Send Backwards is similar to another error handling option, Try Next Alternative, because it affects execution of the branches that go out from a Try step. However, it does so in a quite different way. Its immediate effect is that the steps following the current one are not executed (this is similar to Skip Following Steps). Aside from that, execution of the current branch from the Try step continues. However, the error is also "sent backwards" to the Try step and remembered there, which causes execution to continue with the next branch of the Try step once execution of the current branch has finished orderly. This part is similar to the effect of Try Next Alternative (although delayed), and the same comments regarding the robot state apply.
If the current branch is the last one from that Try step, Send Backwards is not illegal, but results in an error ("all alternatives failed") which is handled according to the way error handling is configured for the Try step.
If there is no prior Try step to be found, the error is "sent backwards" to the beginning of the robot and will be reported via the API and logged just before execution of the robot finishes.
Send Backwards is quite difficult to work with because of the delayed effect, and it is recommended to use Try Next Alternative instead. When robots created using Design Studio versions prior to 8.0 are opened, they are automatically modified to use Try Next Alternative instead of Send Backwards whenever this can be done without changing the results from the robot.