I believe what you are referring to is more commonly called "Critical Speed"
Long stroke engines turn at a very low speed and centrifugal torsional dampers are not effective in counteracting the harmonics developed in the crank shaft.
When an engine is run at a speed that these harmonics synchronize with other vibrations in the drive line the ossilations are amplified to the point damage can be done to the engine.
Troubleshooting 101 "Don't over think it - K.I.S.S. it"