A lot of those "make $$$ working from home!" things are scams like this. They'll offer you, say, $1,000 for a small amount of work. On payday, they'll send you a $1,500 check, along with some excuse for why it's too high, and ask you to send them the extra $500. A couple of weeks later their check bounces, and you're screwed.
Another technique is to just ask you for help transferring money. They might send you $4,000, tell you to keep $500 to compensate for your time, and send $3,500 to their friend/partner/subsidiary outside the country. Then the $4,000 evaporates. This one is especially clever since the original scheme was probably some form of money laundering or other financial crime, so the victim will usually be reluctant to go to the authorities afterwards.
The stereotypical 419 scam, however, has some "plausible" reason why the main transfer has to wait on a smaller transfer in the other direction.
Wait, you want me to type in my username and password into your computer, and then you can take any amount of money from my account, and all everyone needs to do is trust each other?