ANSWER: Putting a Swarm node into archive mode causes the node to change its bidding for SCSP write operations. When a node is empty, instead of bidding low to take writes, it bids high to only take writes if the other nodes are too busy. Essentially this causes the node to try to stay empty. As the cluster fills, the archive node will bid low when it should be bidding high. This causes that node to try to fill itself. Once nearly full, it returns to the normal bidding algorithm.
In a fairly full cluster, setting a node to Archive Mode will cause that node to try to take on as many streams as possible. In most cases, this setting does not prove very useful. It is meant to be used in conjunction with the adaptive power setting feature, such as with an archive cluster, that mainly takes writes instead of reads. You can designate certain nodes to fill themselves to near capacity then spin down to save power.