La Chatelier's principle and pressure equilibrium

[Monkeymaniac]

In TBR, it is stated that when external pressure is increased, partial pressures are also increased and that the displaced equilibrium shifts in a way to decrease the partial pressures, which makese sense.

But then it goes on and say that

In piston reactions, the initial pressure equals the final pressure, if the volume of the piston exhibits no instantaneous change. This is to say that for a piston that starts with a stationary lid, once the lid is statinoary again, the internal pressure equals the external pressure.

I have read this chapter couple of times in the past, but I was always stuck at this point.

When the external pressure doubles (say force on the piston increases by factor of two and is kept constant until the eq. is reached), partial pressure is also doubled (because Pext = Sum of Ppartial, is it right?). Now, once the eq. is reached from this point, Pext is still twice its original value (2*Pexti), but sum of partial pressures are less than 2*Pexti because equillibrium shifted to the side that produced less moles of gasses (La Chatelier's).

Doesn't it contradict what's stated by TBR earlier, that, once the lid is statinoary again, the internal pressure equals the external pressure? Thanks in advance!

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[loveoforganic]

I'm having a hard idea what's being said exactly, but if the pistion (lid) is stationary, the internal and external pressures must by definition be equal, otherwise it would move until they were equalized.