They have a lending platform/bot built into their site. Some people say it is legit, others say it is a straight ponzi, I don't know who is right. All I know is that the cash that comes out is legit, so I don't care if it is a ponzi as I'm not running it so long as I'm not the one left holding the bag at the end, which at this point I'm not. There are several similar sites like I've mentioned. Some, like bitpetite, clearly are Russian ponzis as they fold after a few weeks/months and the money disappears. The seemingly more stable ones, like Bitconnect and Regalcoin, have their own blockchain coin so have some semblance of legitimacy.
Anyway, you acquire some bitcoin and transfer it to their site, usually in exchange for their proprietary coin. They then lend out your coin, and you get a daily interest payment. The base rate averages about 0.8% per day, and then you get a kicker depending on the value of the bitcoin you put in ($1k = 0.1%; $10k = 0.25%). Your principal is locked for a certain number of days depending on how much you put in ($10k = 120 days; $500 = 280 days). So you can't take the principal out, but you can take the interest out daily or you can roll it back in. Assuming you think it has some ponzi elements, the question is how long you think it will last before it collapses, which dictates your withdrawal strategy. The longer you think it will last, the more you reinvest your daily interest profits versus withdrawing them, to get a greater snowball effect.
My friend started with like $1k investment to see if it worked and wasn't a full scam. After seeing it worked, I think he threw in $10k. He fully reinvested for a couple months, then went to like a 50/50 withdrawal/reinvest strategy to get his capital back via the profits. He is currently pulling all cash every day until the end of the year, then will go back to splitting. There is no perfect strategy, just whatever you think works best. I got my principal back via the daily profits and have been reinvesting everything since; I'll continue to do that through the end of the year and then go to a split at that point. It compounds so damn quickly that the temptation is to keep reinvesting, as the numbers get silly really quick.
DISCLAIMER: This is risky as fuck, so don't do it with your grocery money. You have to view this as going to Vegas and not caring if you lose all of whatever you put in. But so far so good; at this point I'm not out anything and I make more interest in one day on what I have in there than I do on all of my checking / money market accounts in an entire year combined.