What's causing my engine to "freewheel?"

What is causing my engine to sometimes rev really high, sort of "freewheel," yet the transmission never came out of gear? If i let off the accelerator for a sec, let the revs come down, then it "re-engages" properly.

It's a cummins. Not sure which one, but can check tonight. Regular roadranger 10spd manual. NOTE: This is NOT a super 10. Just a regular 10spd manual.

Volvo, if that matters.

 

i never thought about this as i rarely use the clutch. As far as i remember, there is no free play. Now, with volvos, and i'm not sure if all trucks have this, but volvos have like a hydraulic resevoir for the clutch and i'm not exactly sure how that works and really only found out about it a few days ago when another driver pointed it out to me.

 

i'll make a note of this for the mechanic. Yes, i'm not an O/O.