A coinflip is as close to a pure bet as skin gambling gets. Two players, two sides, one roll. Player one creates a flip and stakes items or site balance. Player two joins with a stake of about the same value. The site generates a result, usually from a server seed, client seed and nonce, and one player takes the whole pot. The site takes a commission from the pot or from the winner.
How does a coinflip work on skin sites?
The creator picks a side, say terrorist or counter-terrorist, and deposits a stake. The site sets a value range for the second player, often within a few percent of the first stake, so the flip stays close to even. When someone joins, the round runs and the winner receives both stakes. On many sites the odds shift slightly when stakes are unequal: a player who puts in 55 percent of the pot gets a 55 percent chance to win.
The commission is where the house edge lives. A fair coinflip with no fee has an expected value of zero: you win the other stake half the time and lose yours half the time. Add a 5 percent site cut on the pot and each flip has an expected loss of 2.5 percent of your stake. That seems small, but flips are fast and people play many in a row, and a losing streak can wipe a balance in minutes.
What should you know before playing coinflip?
First, variance is extreme. A 50 percent game means losing five in a row happens about 3 percent of the time, which is often enough that most regular players will see it. Second, item values on the site may not match what you could sell for, so a "fair" 50 dollar versus 50 dollar flip may really be 45 versus 50 in resale terms. Third, provably fair verification proves the winning side was not changed after both players joined, and nothing more. The edge is the commission, and it does not go away.
If you want to see how flips play out without risk, use the case battle simulator and the house edge calculator, and read responsible gaming for limits before playing for real.