You pay one single buy in, and for this you get a stack (say 6000 chips). By default, you get only 3000 to start playing with, but can take the other 3000 at any time up to the first break - usually 90 mins. This enables you to play poker early on without putting your entire tourney at risk...... Or, you could take the second half of your stack right at the start and try to "own" the table.
It"s my favourite form of poker, mixing the loose aggresive nature of the early stages of a rebuy, with the more reserved play of a freezout.
As for tactics, I tend to just take half of my stack early on to minimise risk, and try to play standard ABC poker until the break...... there are usually enough playing loose and wild such that you will get paid when you catch a hand..
After the break, all unclaimed second chance chips are delivered, and the game becomes a standard freezout.