Lets say you're unsophisticated. You say "I like my 1080ti, so I'll buy some NVIDIA stock." I'm a tech stock expert, I know NVIDIA is on a downward slope because its over-valued right now. I tell you "Sure kid, here's a promise for 100 shares at today's price of $50 each." I take your $5,000. Everyone is happy. Joe Investor doesn't actually have the stock in his possession just yet.

I buy the stock myself a few days later, but at $40 each for a grand total of $4,000, and I just made $1,000. You get your 100 shares, I make $1,000 plus fees. Now imagine doing this hundreds or thousands of times a day. Or I simply sell my own existing supply conveniently to my own buyer and I dictate the fees, which has value in itself. Sure I'm only making $5 per head per month with these low-end customers, but now I have 100,000 customers instead of 1,000.

A savvy investor wouldn't ask to buy these shares. He'd know that he's better off shorting himself. Unsavvy investors are all over the place and if you can get them to come to you, you can make a lot of money off them with minimal investment or risk.

On top of that you can offer them predatory loans for investing. This app offers loans, no idea on terms, but I can't imagine them being very nice. They put up collateral like their car or home, so you have no risk. They fuck up, or not, it doesn't matter, you're at least getting fees and probably screwing them in other ways. Oh and you're getting interest on that loan as well.

edit: Front Loading absolutely happens, it can be done in a way that's difficult or impossible to detect and has a lot of loopholes as well.