I think what Rowuk says is right, it may not be so much the reverse leadpipe but the different bracing the makes the horn "feel" easier to play. The same thing with tunable bell horns also.
When more energy makes it from the mouthpiece to the valve block the horn resonates more freely (actually, the air column does). Different bracing techniques can be used to limit the amount of 'loss' that the air column experiences along the way to the valve block.
The loss can be from vibration leaving the leadpipe and moving toward the bell or vice versa. The most effecient leadpipe would have no bracing at all, but obviously would be impractical.
I have experimented with bracing myself. My current every day Bb trumpet has a leadpipe sleeve and brace system that isolates the leadpipe and I can attest that it does make a big difference compared to normal bracing.
It is like someone put a turbo on it, so to speak!
GZ