The antenna tuner, when adjusted properly, creates something called a "conjugate match", which is technical jargon for letting the power from one side (the radio/amplifier side) pass while reflecting the power coming in from the other side (the antenna side) back towards the antenna. This is what happens to that extra power that is reflected from the antenna, namely it gets sent back towards the antenna. If you want to know more I highly recommend looking up a freely available .pdf file called "Another Look at Reflections" by M. Walter Maxwell.
The reason ladder line and other forms of balanced feed line is used is when it comes to losses is the worst of them, 300 ohm twinlead, has about the same loss characteristics as what some see as the "almighty" LMR-400, and the best, a well made ladder line, makes LMR-400 look like RG-58 in comparison. When working with antenna tuners where there is a high SWR between the antenna tuner and the antenna, feed line losses also increase, so this more efficient feed line and its better efficiency pays back in spades.
Most antenna tuners aren't actually inefficient, unless they are used near the edges of their limits, or you overpower some component within it. At worse, they are only slightly less efficient than the specialized antenna tuners at the feed point of antennas (ok these are more commonly called matching circuits, but they do the exact same thing as antenna tuners in the exact same way, and in fact are sometimes the exact same circuits.)
Antenna tuners do have some losses to heat, but even when running large amounts of power they don't tend to warm up enough notice, so I woudn't worry about it unless the case the antenna tuner is in warms up noticeably.
The DB
