What efficiency principle did James Watt introduce via the concept of the steam cutoff?

Allowing the steam to expand after the piston achieved sufficient initial velocity, rather than pushing for the entire stroke

Watt continuously sought methods to maximize the conversion of heat energy into mechanical work, leading to the concept of the steam cutoff. In less advanced engine cycles, steam was admitted throughout the entire duration of the power stroke. Watt reasoned that once the piston had built up significant velocity due to the initial burst of high-pressure steam, the steam pressure itself was no longer the most economical driver for the remainder of the stroke. By cutting off the steam supply partway through the stroke and allowing the trapped steam to continue expanding into the vacuum, the engine could extract further useful work from the same initial charge of steam, thus greatly increasing the overall thermodynamic efficiency.