Streams live posts from Jetstream v2 — see also the Bluesky Protocol Services announcement.
Example message to filter for only specific DID handles (resolve those here):
{
"type": "options_update",
"payload": {
"wantedCollections": ["app.bsky.feed.post"],
"wantedDids": ["did:plc:kft6lu4trxowqmter2b6vg6z"],
"maxMessageSizeBytes": 1000000
}
}