Second touch (2T) is a feature on organ manuals and pedals that activates a second set of electrical contacts when the keys are pushed down with a bit more pressure.
This feature is activated when the keys move about 1/8 inch further down past the first touch position.
Second touch stops are provided for each keyboard that has the second touch feature which, on a Wurlitzer theatre pipe organ, are supplied by shorter stop tablets located on the straight fall board immediately above the uppermost manual.
The photo shows an example with 4 labeled second touch stops (left) controlled by the Accompaniment manual and 3 unlabeled second touch stops (center) controlled by the Solo manual.
Additional second touch stops (extreme far left, off the photo) would be controlled by the pedal.
Typically, the bottom two manuals and pedals of a Wurlitzer will have second touch, therefore both manuals and pedals in a two manual instrument will have it.
Second touch stops do the same job as the first touch stops for the second touch key contacts.
This can be demonstrated by turning on some stops for both first and second touch, using a fairly light registration on first touch and picking out something for second touch that will stand out when added to the first touch stops.
By playing deeper into the keys we can then hear the 2T stops being added.
This feature is useful for, among other things, adding rhythm to a sustained chord, creating a counter melody by using it on individual notes of a chord, layering of sounds, or accents.