As other have said, there are no principal repayment requirements unless you reach the limit of the margin you are allowed. Be wary that your debt amount is fixed, aside from accruing interests, (say 100’000.-) but that, unlike a house, the value of your assets moves wildly. Your assets can loose half of their value overnight (well, not quite overnight but pretty quickly) so what could have been a 100% margin (100’000.- of debt, 100’000.- of assets) can become 200% (100’000.- of debt, 50’000.- of assets) without you doing a thing, resulting in a margin call.
If your margin is called, you’ll have a few days to cough up the difference in money. If you can’t, your assets are sold at their low price to get you back into “acceptable” margin territory. This can happen again and again if the price of your stocks keep falling.
Note that margin becomes dangerous when stock prices drop. Big stock price drops tend to happen when other bad things happen in society, meaning two things:
you could be out of a job, if you’ve loaned the money, the person you’ve loaned it to could not be able to repay it anymore, the value of your house could be reassessed (dangerous if you have a mortgage too), the value of your other assets may decrease too, making it a very bad time to sell anything. In short, you could be in financial troubles already ;
the situation may be too dire with too many people on margin so that the broker needs to adapt its margin requirements, either lowering the total margin available (you’d get called when you thought you were safe) or the interests due (your margin becomes suddenly unsustainable).
Margin loans are part of the things that go well as long as all goes well but can get really, really, really bad when things get bad. Apply caution. MMM is a very enthusiastic person and, in this case, writes in a genuinely reckless way without emphazing the consequences margin play can have. That he just discovered a tool that’s been there for ages may mean he hasn’t truly internalized what being margin called really means when you don’t have the money to meet the requirements. He, himself, has got the money, after all. Be wary if you’re not a multimilionnaire yourself and are borrowing more than you can repay in a blink.
I’d read Market Timer’s cautionary tale before going on margin: A different approach to asset allocation - Bogleheads.org