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@DFP
Felt like adding some more specific detail.
You'd get more "burn" because in a typical out of seat position the riders body moves forwards. This puts their hips either directly above or slightly in front of the cranks. The further forwards you are the more difficult it is for hamstrings to contribute. The primary mover becomes the quads extending the knee, and whats more is that your body weight is supported on the tension of the quads because the knees are left totally unlocked. So the quads are getting heavily fatigued by this supporting role even while it is the opposite leg pressing down.
If you actively push the pedal forwards at the top of the revolution, the motion is similar to kicking. Again, the quads are heavily worked to begin extending the knee at very difficult leverage.