They basically bounce the sound waves off a parabolic mirror, so that they come from multiple directions but converge on one spot. The spot where they converge is the only place where the sound waves are strong enough to cause cavitation.

This is actually a fairly mature technology that has been used to treat kidney stones for decades (lithotripsy).

This is my non expert opinion but based on what I know about sound waves I think this is how they are doing it. Hopefully someone more knowledgeable can chip in with more sources.

It looks like you can target and focus the sound waves to converge and only be strong enough to trigger the cavitation effect in tumorous areas . The cavitation creates tiny bubbles and damage the tissue only in the focus area.

It's similar to how radiotherapy works in that it's concentrated damage in a targeted area, but less much less damaging to other tissue that it passes through.

Sound waves shouldn't be as harmful as radiation sources, so it could be a great addition to the arsenal if it proves effective.

The sound waves are focused onto the tumor, so they only have enough energy to damage tissue there, and not anywhere else.

Think about focusing the sun's rays through a magnifying glass; they're only concentrated enough to burn at a very small area. The focused ultrasound is similar, although they're probably using some form of phased array to get the equivalent effect.