The protocol works like this:
When you do a BTCX operation it charges an x% on your transaction. The estimated amount to be charged will shown to each user (doing the BTCX operation) at the moment they do the operation. The % depends on the demand of the BTCX. If no BTCX issued, the % tends to 0. The % grows with BTCX demand.*
That total interest (rBTCs) goes to a bucket (the interest bucket). You can see the total amount of that interest that had been charged to all users in current settlement period in the metrics section.
Each day a portion of that interest bucket goes to the BPro bucket. All the interest in that bucket will be moved to the BPro Bucket by the end of the settlement date.
For example, in current settlement period the total amount of rBTC charged by the protocol to BTCX that was still not assigned to the BPRO Holders is 0.018389. Each day a proportion of that rBTCs will be moved to the BPro bucket, appreciating the technical price of Bpro.
*An additional detail, although the demand for btcx increases the rate, also the scarcity of “free DoCs” increases the rate charged.
For example, if no one does new BTCX operations, but a user redeems DoCs, the fee for making new btcx operations will increase.