The idea is that congress sets a standard and set of parameters for negotiation that the US Trade Authority then goes out to negotiate and structure trade deals. They must stay within the boundaries set by congress. Then they can go out and negotiate in good faith, knowing that if they stay in those boundaries, congress has agreed to debate and then approve or reject a deal within 90 congressional days (about a year). The bar for rejection is high, because the deal in front of them has to already fit within the parameters congress themselves set out.
The term "Fast Track" has to be one of the most misleading terms used in political lingo. Fast Track in this sense basically means almost a year for congress to approve a trade deal.
Moreover, very little about TPP or Fast Track is "secret" or "backroom" and is how trade deals have been done for the longest time. It's unclear why the EFF is taking up this cause.
This is also helpful, and is non-partisan congressional research funded by congress to surface the real facts: https://fas.org/sgp/crs/misc/RL33743.pdf
The amount of propaganda being spun up over TPA is surprising, considering we've had it for 30 years and it's been an economic boon for the US economy.
Edit: I have no idea why I'm being downvoted for this.