By querying using a ranged regex an additional indexBound is used with the part of the regex until the range begins.
For example:

db.coll.find({ _id: /^1541\/1\/F\/8\/1\/2014\/0[8-9]/ }).explain("executionStats")

the indexBounds are:

{
    "_id" : [ 
        "[\"1541/1/F/8/1/2014/0\", \"1541/1/F/8/1/2014/1\")", 
        "[/^1541\\/1\\/F\\/8\\/1\\/2014\\/0[8-9]/, /^1541\\/1\\/F\\/8\\/1\\/2014\\/0[8-9]/]"
    ]
}

This way it scans a lot more from the index. In this case the _id represents a date prefixed with other info. So the "additional" indexBound will scan all 01 to 09 months.