Great question there Tye. What many mean by Lugging Power, is how well the old 2 cylinder Deeres can drop to very low RPM's, yet still work and pull a load and deliver good torque to the rear axles. The reason is they have a HEAVY FLYWHEEL (stores energy) and LONGGGGGGGGGG STROKE and the longer the stroke the longer the lever arm which means more torque (turning/twisting power). Its like when you use a cheater pipe for a long lever arm and apply more torque to a bolt head. With a long stroke, theres a long distance between the cranks center out to the the connecting rod throw, so as the piston pushes down, theres a lot of torque at the cranks center.
Next, Horsepower is a function of Torque X RPM. On the high RPM engines (like Olivers and IHC's etc) much of the derived HP is due to the high RPM's instead of the torque, therefore, as RPM drops, HP drops rapidly and with their short strokes, low end torque aint so hot.
HOWWEVER, the JD's are already at relatively low RPM (compared to Ollies) so when it drops off some, the low end torque is still good and the HP is still decent compared to even at their max RPM. GOOD LUGGERS
I hope this answers your question. Feel free to e mail or call me with any more questions.
Good Luck n God Bless ya
John T Nordhoff, Retired Engineer in Indiana