The Technical Committee(TC) was introduced in the Kusama rollout and governance post as one of the three chambers of Kusama governance (along with the Council and the Referendum chamber). The TC is composed of the teams that have successfully implemented or specified either a Polkadot runtime or Polkadot Host. Teams are added or removed from the TC via a simple majority vote of the Council.
The purpose of the TC is to safeguard against malicious referenda, implement bug fixes, reverse faulty runtime updates, or add new but battle-tested features. The TC has the power to fast-track proposals by using the Democracy pallet, and is the only origin that is able to trigger the fast-tracking functionality. We can think of the TC as a "unique origin" that cannot generate proposals, but are able to fast track existing proposals.
Fast-tracked referenda are the only type of referenda that can be active alongside another active referendum. Thus, with fast-tracked referenda it is possible to have two active referendums at the same time. Voting on one does not prevent a user from voting on the other.
A proposal can be canceled if the technical committee unanimously agrees to do so, or if Root origin (e.g. sudo) triggers this functionality. A canceled proposal's deposit is burned.
Additionally, a two-thirds majority of the council can cancel a referendum. This may function as a last-resort if there is an issue found late in a referendum's proposal such as a bug in the code of the runtime that the proposal would institute.
If the cancellation is controversial enough that the council cannot get a two-thirds majority, then it will be left to the stakeholders en masse to determine the fate of the proposal.