The phrase "torque curve" refers to how much twisting power an engine produces at different speeds. It shows how the engine's torque (or rotational force) changes throughout its range of operation. Full definition
The upgraded engine in manual cars gets a flatter torque curve as well, helping improve responsiveness. (kbb.com)
... and here is an interesting print out of torque curves, comparing a 40 mm length with a (ridiculous, but interesting) 330 mm length. (mechanics.stackexchange.com)
, respectively, while sporting a broader torque curve for more immediate and powerful acceleration response. (chicagoautoshow.com)