Most 3-phase induction motors are designed to have a reasonably high locked-rotor torque (130-160%), a high breakdown torque (150-250%) and low slip (1.5-3%). A motor with a high resistance rotor has a high locked-rotor torque, but high slip and no breakdown-torque inflection point. A motor that has a low resistance rotor has low slip and high breakdown torque, but the locked rotor torque is lower than would be desirable.
A rotor bar that is close to the surface of the rotor has a low leakage reactance reactance while a rotor bar that is closer to the shaft has a higher leakage reactance. If the rotor is constructed with two rotor bars, or one elongated bar that extends toward the shaft, the effective rotor resistance is high at locked-rotor and low at the normal operating point. That happens because the rotor frequency is high at high slip and low at low slip. At locked-rotor, the rotor current is forced to flow mostly in the low-reactance part of the rotor bar near the rotor surface. The effective resistance is increased by reducing the bar area in which the current flows.
As the motor accelerates, there is a transition between high rotor-resistance characteristics and low rotor-resistance characteristics. That transition can result in an inflection in the torque vs. speed curve.
There is a Chapman text that explains and illustrates this quite well.
Here is a link.
Pull-up torque is not the name of a special or undesirable phenomenon. It is a standard part of an induction motor performance specification. The NEMA definition is:
Pull-up torque: The minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. For motors which do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed. [MG 1-1.48]
For some motors, pull-up torque is equivalent to locked-rotor torque. For others, pull-up torque is less than locked-rotor torque and occurs at a speed between zero and the speed at which breakdown torque occurs. NAMA standards allow pull-up torque to be less than locked-rotor torque and, for some ratings, less than rated torque.
It is clear from the above linked reference and from similar references that rotor bar shape, position in the rotor and material are the primary factors used to shape the torque vs. speed curve of an induction motor.
Harmonic torque is generally minimized by several aspects of motor design. One factor is the selection of the number of rotor bar slots relative the the number of stator winding slots. Another is "skewing" the rotor bar slots at an angel relative to the shaft direction. A third is the distribution of the stator windings to produce a sinusoidal flux wave.
One undesirable effect of harmonic torque is a distinct dip in the torque vs. speed waveform as shown below. That is different from the relatively gentle inflection in the torque curves of some production motors.