There's actually quite a bit more to rshares than that. rshares are not unique to Steem Basic Income. Actually they are the base unit that Steem uses to calculate rewards. (the r stands for 'reward')

When you upvote a post, your front-end client tells the witnesses your upvote %. The witness checks your VP% and mVests (SP) and calculates the actual rshares of your upvote. Those rshares are stored by the witnesses as meta-data for the post. You can see all the rshares rewarded to a post on steemd, by changing the 'it' in any steemit post url to a 'd' and then clicking 'Vote Details' below the raw markup for the post.

for example:

(img capture from https://steemd.com/contests/@steembasicincome/steem-basic-income-faq-contest-500-sbi-in-prizes)

When your post pays out, the witness checks the total rshares rewarded to the post against the total rewards pool (pending rewards not yet paid out) and converts your rshares into SP/STEEM/SBD.

Since the total rshares that that you can vote are directly driven by your mvests (SP), they are the most accurate possible unit that we can use to calculate the upvote values that we owe our members. As such, we use them for all our balance tracking. Each unit (and bonus unit) earns 80M rshares every 2.4 hours (800M per day, 5600M per week).
That amount would round to 0 if we used SBD or SP. When we started our program, STEEM was at $8 and 800M rshares per day was enough for an 0.01 upvote every day. Now it's only enough for an 0.01 upvote every 25 days (which is why we recommend 25 units to get a daily upvote). If STEEM price continues to rise, the value of rshares will go back up and the SBI needed for a daily upvote will go down.

Your rshares rewards are stored in our database as a 'pending upvote balance'. Each member also earns 1.34x multiplier on their upvotes. This part you got dead on - you give us rshares through your votes, we multiply those by 1.34 to cover the curation we will get from our returning vote, and we add them to your balance to give back to your over your next few posts.

When we deploy our system to redistribute outstanding vote balances from inactive and blacklisted accounts, that will be calculated using rshares too.

You are welcome to edit your submission and I will review again (or you can resubmit if your edit is substantial.)

Edit: Technically accurate. Response accepted as a qualifying submission.

Don't forgot you can also earn points by scoring the clarity and helpfulness of other qualifying responses.