What does DNG SPARQL pre-count option do?

I have noticed in the past that the pre-count option makes the query take much longer (from 3 to 5 minutes to over an hour) and most of the time is part of the pre-count process. Here is an example: 

With pre-count: 
Pre-count Execution Time: 3777 s 
Query Execution Time: 1 iterations, 194 s, 194 s avg 

Without pre-count: 
Query Execution Time: 1 iterations, 142 s, 142 s avg 

Both query results were the same. 

I couldn't find pre-count mentioned on the Apache Jena site or elsewhere any mention of what "pre-count" does in relation to SPARQL? Is this a specific thing to IBM products? to the DNG Debug Page? 

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Update, gathered some timing data of how long it takes to run when pre-count is selected and when it isn't selected.

------------------------------------------------------------------------
|Run   |          No Pre Count           |            Pre Count        |
------------------------------------------------------------------------
|      |Pre-Count    |Query    |Total    |Pre-Count    |Query    |Total|
------------------------------------------------------------------------
|1     |0.00         |27.60    |27.60    |24.16        |26.75    |50.91|
|2     |0.00         |26.13    |26.13    |23.46        |26.54    |50.00|
|3     |0.00         |24.91    |24.91    |23.64        |25.97    |49.61|
|4     |0.00         |25.55    |25.55    |22.72        |25.00    |47.72|
|5     |0.00         |26.03    |26.03    |22.77        |25.93    |48.70|
------------------------------------------------------------------------

</pre>
This is the SPARQL query used (from&nbsp;<a href="https://jazz.net/wiki/bin/view/Deployment/CalculatingYourRequirementsMetrics">https://jazz.net/wiki/bin/view/Deployment/CalculatingYourRequirementsMetrics</a>)
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PREFIX dc: <http://purl.org/dc/terms/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX rm: <http://www.ibm.com/xmlns/rdm/rdf/>
PREFIX rmTypes: <http://www.ibm.com/xmlns/rdm/types/>
PREFIX rrmReview: <http://www.ibm.com/xmlns/rrm/reviews/1.0/>
PREFIX nav: <http://com.ibm.rdm/navigation#>
PREFIX jfs: <http://jazz.net/xmlns/foundation/1.0/>
PREFIX owl: <http://www.w3.org/2002/07/owl#>
SELECT DISTINCT ?context ( count(?artifact) as ?artifactCount ) ( count(?binding) as ?bindingCount ) ( count (?link) as ?linkCount )
WHERE {
{?artifact rdf:type rm:Artifact .
OPTIONAL{?artifact rm:boundArtifact ?boundArt .}
FILTER (!BOUND(?boundArt)) .
?artifact jfs:resourceContext ?context
}
UNION
{?binding rdf:type rm:Artifact .
?binding rm:boundArtifact ?boundArt .
?binding jfs:resourceContext ?context
}
UNION {
?link rdf:type rm:Link .
?link jfs:resourceContext ?context
}
}
GROUP BY ?context
ORDER BY ASC (?context )