Ellipsis

Auxiliaries can appear alone where a main verb has been omitted, but is understood:

I will go, but she won't [go].
The verb do can act as a pro-VP (or occasionally a pro-verb) to avoid repetition:

John never sings in the kitchen, but Mary does. (pro-VP: replaces sings in the kitchen)
John never sings in the kitchen, but Mary does in the shower. (pro-verb: replaces sings)