Is it okay to submit manuscript to arXiv after "revise and resubmit"? I've read this question about how some publishers (e.g., Springer) allows authors to submit to arXiv before journal submission, but they make authors wait for a year after acceptance to submit to arXiv. 

My question is: Is it okay to submit to arXiv during the time when the paper is under consideration by the journal, for example after it has been "revised and resubmitted" but before final decision?
The version you mention is called a postprint. Publishers have various policies about them. You can use the SHERPA/RoMEO service to get a summary of the policy in place for a particular journal.

Here is an example of what you get for Lecture Notes in Computer Science, edited by Springer:



As noted in the comments, you do not necessarily need to abide by these policies, for instance if your funder has set up an open access policy.