Torque is the size of punch an engine can deliver, as opposed to its power, which is the rate it can throw those punches. It's a measure of turning force of the engine. But this 'final' torque, as delivered from crank to gearbox, and the rear wheel, is a composite of two types of torque.
The first is called combustion torque and it's generated by the downward push of the piston on the crank. It' under the direct control of the rider, in that more throttle equals more combustion torque. In this sense it is linear. But this 'pure' torque is blended with another, non-linear torque, generated by the change in inertia of the spinning crank.
Imagine a spin dryer with a large, single lead weight in it – the drum would spin with an exaggerated, lopsided motion. It might spin at the same rate as an equally loaded but evenly weighted drum – but within a single rotation it's accelerating and decelerating. And as the revs increase, the vibration increases.
Some engine builders say a spinning 180 degree inline four crank, driven by opposite pair of pistons rising and falling, behaves in a similar way – broadly speaking, you'd need two weights in the spin dryer, but it's the illustration that matters. Incidentally, this is the same mechanism that produces secondary vibration, caused by the difference in acceleration of the pistons rotating in opposition.
This lumpy rotation introduces a new, non-linear, noisy torque – called inertial torque – to the combustion torque. The sum of the two is called composite torque and it's this that arrives at the rear tire. According to many, that is a problem, because at high revs the noise of the fluctuating non-linear torque is greater than the pure signal of the linear combustion torque. And the higher the revs, the greater the noise.
Either way, the interference of inertial torque leads to a disconnect between the throttle and the rear wheel, hampering the rider's ability to accurately modulate throttle control at high revs and high angles of lean.
Some motorcycle manufacturers believe that they found a solution to the problem, crossplane crank setup. A crossplane crank, with its four 90 degree crankpins, is like having four weights in the spin dryer – you get a much smoother spin cycle and, indeed, a crossplane crank has near perfect secondary vibration, although it needs a balancer to cope with a twisting couple vibration unique to the crossplane design. And you also get almost no inertial torque to interfere with combustion torque. It's likely that more and more motorcycle manufacturers will move into the direction of the crossplane crank setup in the near future.