Originally Posted by
thump55
Clipless may not "significantly" increase power output, but it allows you to maintain the same power output for a longer time, which is in all reality, an increase in power.
By using other muscles besides the main "push down" group, you spread the burden around which makes all the muscles involved last longer.
Not really. The point is you can still use those other muscles to unweight the non-power leg with platform pedals. It's not necessary to apply any force to the rising pedal.
Spreading the work to other muscles sounds great but doesn't result in any extra power as the main 'pipe' delivering oxygen to your muscles is the primary limiter. That's why EPO works. Send more oxygen to your muscles and they can do more work.
Bacciagalupe's summary was accurate.