Yep. They shut down the slot adjacent to the one with the sounding reed so as to channel all air towards the latter and reduce the effort needed to make the reed sound. They can be a real monkey for harmonica players because the air of lung origin blown towards the reeds (unlike in a squeezebox) is moisture-laden. A cold harmonica will result in much condensation on both the reed and the valve, causing all sorts of problems such as popping and sticking, not to speak of providing fertile, moist territory for bits of chip and pork scratching to adhere. That's why it's a good idea to warm the harmonica before playing it.