Back and fade
See a call you like? Back it. Think they're wrong? Fade it. Either way it's your own position, with your own money, at today's price.
Back
Back puts you on the same side. You get in at the current price, not theirs, so if the market has moved since their call your payout is different. The feed shows both prices so you can decide.
Fade
Fade puts you on the other side. It works on two-way markets, Yes/No or two teams. On a market with a draw there's no single other side, so Fade isn't offered.
Where you can do it
Back and Fade sit on every call: in the Friends feed, on the market and call pages, in crew chat when a call lands there, and right on an alert. So when someone you've got the bell on for backs a call, you can take a side without leaving the page.
Your stake, your price
Back opens the call panel with your default stake, not theirs. Their stake shows on their call, so you can see what they put in, but yours is your own. Change it before you confirm.
Who gets the credit
The person you backed or faded gets an alert, and backs count on their profile (“others backed their calls 1.4K times”). One only counts if it's a real filled order on the same market and side. The server checks.