10 buy N shares @ $0.011
20 sell 500 milliseconds later at $0.014
30 goto 10
well that's pretty much simplified. but the idea is to make profits from really small price differences very frequently. the hard part is to chose the right stock and do the trade fast enough.
the first part is solved by supposedly smart algorithms (the guys who invent those algorithm are called quants because ... well ... the first guys who started HFT were ex quantum physicists. nowadays the term got coined to quantitative analyst)
the 2nd part of the problem is the execution of a trade in the lowest possible time span. that's where our coder guys come in - they implement the algorithms and trade stuff and make their software as fast as possible.
if you ask me this is all big bull shit. it's like a high speed rand() - one time you get lucky one time you don't. and the $100k day/profit is an average. you don't know how large their trade volume is (I guess it's way more than $100k - way way more). so on one day in a year they might hit the jackpot and their rand() chose the right stock at the right time and make like $50 mio and then the rest of the year they only hit shit losing $20 mio. at the end they made $100k/day ...
in my app I am opening the auth page in a web view and then parse the html of that web view to get the pin.
an alternative is to register a custom url scheme like myapp:// but for that you have to email twitter's api team