Direct-drive exploits errors by making their consequences inescapable. You can sit up, 'sit-in,' or slow down. It doesn't matter. If the wheels roll, the cranks turn. A fixie will insist that you learn to spin smoothly, or soft-pedal home in aching frustration.
Poor technique is often proportional to velocity.
Acceleration will ultimately be met with disproportionately larger energy expenditures. You're going faster, but the fatigue factor is rising even more rapidly due to decreasing efficiency. On a fixed-gear trainer, the only way to maintain a quick tempo is to become smoother. Pure form becomes the only 'fix' for an addiction to speed. Direct-drive compels you to actively manage continually moving crankarms.