Weird mono plug

[Psych0F0x]

What's up with the extra thingy touching the whatchamacallit???   What does it do and what do I connect it to?
I got them from banzai effects..

[southtown]

its a switch that breaks when you plug a jack in, could be useful for a external footswitch or something, so that when not plugged it shorts the jack

[sfr]

A "normally closed" jack - very useful when putting multiple things in the same enclosure - connect the output of and effect to the tip as usual, and then connect that extra tab to the extra tab another one of those jacks.  On that second jack you connect the input of the effect to the tip as normal. 

With nothing plugged in those jacks, the output of one effect feeds into the input of the second - but insert a plug into those jacks, and the connection between the tips and those extra tabs breaks (the tip flexes against the jack and breaks contact with the extra tab) and now the signal goes from the output of one effect to whatever you places at the end of that cord, and back through to the input of the next effect through that patch cable.  (Or not through that that next effect if you don't plug anything into the second jack.

Same idea as how effects loops and send/returns on amps often work - one of these jacks attached to the output of the pre-amp, the other attached to the input of the poweramp.  Normally, they're connected through the extra tabs on the normally closed jacks, but if you insert a plug into the jacks, those connections are broken and you can insert things inbetween the two or take the out directly from one, or feed directly into the the other.

You can also use the same principle to bypass a pot wired as a variable resistor with an expression pedal simply by plugging in a jack. 

Hope that made sense, I'm not the best at describing things.

[MKB]

You also see these by the score in guitar amps.  The extra leaf contact is connected to ground so that the center contact is grounded with no plug installed.  When you plug in a cord, the ground contact is broken.  This way noise is lowered when an input is unused.  The same principle is also used on speaker output jacks, as most tube power amps won't blow up as fast if they see a short than if they see an open contact (assuming no speaker plug installed). 

[southtown]

You could also use it for headphones, make it turn the speaker off when you plug them in

[user]

These are also called Input Short Jacks or something. I use these for all of my effects. I currently used a switchcraft jack for the   input of one of my builds.

[blanik]

i used those for the second output of stereo effects.... when used in mono, the "switch" on this jack is closed so the signal goes back to the board to be mixed in mono, when the second amp jack is plugged, the switch opens and the wet signal goes out there...

R.