They have two ports to allow air in, one for outstroke and one for instroke.
From the en.wikipedia.org
On instroke, the same relationship between force exerted, pressure and effective cross sectional area applies as discussed above for outstroke.
From the en.wikipedia.org
Typographers also speak of an instroke, where one starts writing the letter, as at the top of a c f, and an outstroke, where the pen leaves off, as at the bottom of c e j k t y.
From the en.wikipedia.org
More examples
The stroke of an engine piston moving away from the crankshaft