As explained by Ben Lilliston in a recent interview, Fast Track (also known as trade promotion authority, or TPA) gives the President the right to negotiate and finalize a trade agreement, which Congress then votes on in its entirety. And, once the agreements are completed, you can no longer make any amendments.
Under the US constitution, Congress has the right to engage in trade agreements prior to this, and to set negotiating parameters. Under Fast Track, Congress forfeits this right, allowing the President to negotiate at will instead. According to a recent press release:22
"Rep. Peter DeFazio (D-OR) today spoke out against a provision buried in trade promotion authority (TPA) legislation that could help a government or multinational corporation attack state or national laws that require the labeling of genetically engineered foods.... The Obama administration is asking Congress for fast track authority in order to negotiate the Trans-Pacific Partnership (TPP) and the Transatlantic Trade and Investment Partnership (TTIP), a free trade agreement with the European Union.
'Call it the smoking gun,' said DeFazio. 'Proof that fast track and massive free trade agreements like the Trans-Pacific Partnership are written by and for multinational corporations such as agriculture giant Monsanto. Instead of using trade deals as an opportunity to protect and strengthen consumer rights by joining the countries which require genetically engineered food to be labeled, this administration wants to benefit wealthy corporations at the expense of the public.'"