The math presumes, reasonably, that slowing the engine down does not bring it below the speed where the torque curve begins (generally a function of overlap and intake valve closing - higher number for either raises the minimum RPM), or your stall speed (with automatic).
Stock cam + low stall or stick will run acceptably at under 2,000 RPM, hot cam and 2,500 stall won't.
Locking converters with computer sensors are very fussy about this, and will ratchet in and out of lock-up at certain RPM and throttle positions.
A V8 is probably safe, but some L6s are not happy at a specific point between 2,100 and 2,500 RPM (depending on the engine design) - it's a harmonic period. If it feels "buzzy", change the speed slightly.
