Traditionally, the wah-wah effect was activated by a mechanical latching switch of the type shown below, located under the pedal toe. Pushing the pedal all the way forward and applying a little additional pressure would engage the filter. The Vox, Crybaby, and many more use this arrangement even to this day.
There are disadvantages to the design however. Firstly, the switch produces mechanical noise in the form of an audible click when operated, although this is not really a problem when playing at volume. More seriously though, when the pedal is activated, the wah filter will be by definition in its fully open position. To counter this, Morley decided to place the switch to one side of the pedal, allowing the effect to be engaged with the wah frequency in any desired preset position - particularly useful if you are using the effect as a kind of fixed tone control.
Morley Power Wah
Note the switch on the right side of the unit.
Another issue with the conventional switching scheme was that even when the effect was bypassed, the guitar signal was still passing through the internal circuitry. This was a cause of worry for many players as they perceived that the guitar signal was being degraded in terms of level and high frequency content - a concern which was largely unfounded, as any losses there were could be compensated for easily by making minor adjustments to the amplifier's pre-amp and tone controls.
Nevertheless, some purists simply had to have a 'true bypass'. This involved configuring the switch such that when the effect was 'off', the internal circuitry was completely excluded from the signal path. Nothing is perfect though, and true bypass arrangements bring another problem with them - when switching, an audible electrical click can be heard, which at high gain levels can be intrusive - you pays your money and takes your choice! Guitar technicians often offer true bypass functionality as a modification for many older pedals, and some modern off-the-shelf models also feature it, with their designers using some ingenious techniques to minimise the switching noise.
Finally, a third option has become popular - namely FET (Field Effect Transistor) switching. This is a switching process in which two FETs act like solid-state relays, and are able to noiselessly switch signals both mechanically and electrically - the physical switch is a simple momentary action type, as the latching is done electronically. The signal does have to pass through the FETs, but degradation from this is negligible. Boss pedals have always used FET switching, and other companies have since followed suit. It's worth noting that the switching scenarios discussed only relate to pedals using analogue technologies - things are rather different when you go digital, as we shall see later on.