They have two ports to allow air in, one for outstroke and one for instroke.

The sound of her wet flesh suctioning him back in on every outstroke aroused her just as it always did.

The valve opens again in time for the piston to expel the burnt gases before the next outstroke begins.

On instroke, the same relationship between force exerted, pressure and effective cross sectional area applies as discussed above for outstroke.

(Its outstroke serifs are one-sided, but they don't curve up.)

Her cheeks expanded obscenely on the instroke and hollowed on the outstroke, as she labored with mouth and hands.

Instead they usually have one-sided serifs that curve up on the outstroke (contrast the flat two-sided serifs of a roman font).

The waiter was outside sweeping the street, and you could hear the scratchy hiss of his broom against the sidewalk, heavy on the outstroke, lifted clear on the instroke.

At the beginning of an outstroke, a valve in the head of the cylinder opens and admits a charge of burning gas and air, which is trapped by the closing of the valve and expands.

Due to the effective cross sectional area reduced by the area of the piston rod, the instroke force is less than the outstroke force when both are powered pneumatically and by same supply of compressed gas.