The "En Prison Rule" is very similar to La Partage Rule (literally "in prison"). You'll find this at some French Roulette tables (say in Monaco). We have never seen it online (this doesn't mean there isn't a casino that offers it, just that we haven't found one. Let us know if you find one.
If you play at a casino that offers this rule, you can get the roulette house edge down to 1.35% - the same low percentage as you'll get playing at an online roulette game that offers "La Partage", so essentially you don't need to worry about searching too hard for this.
So what's the difference? Well, both rules concern playing even money outside bets like red/black or the odd/even bet on a French Roulette table. In La Partage, you get half your bet back if the ball happens to land in zero. If you play on a table that offers "En Prison", the dealer will place a marker over all even-money bets should the ball land in the zero pocket, effectively putting the bet in prison for the next spin. You repeat the bet, and if it comes in your original bet is returned (you don't get paid 1:1 and get your original bet back). The reduction in house adge is exactly the same as "La Partage". This can come in handy for systems such as the Single Column Even Bets strategy which involve the even money bets.
At some casinos, there is a further twist to the En Prison rule: a "second spin zero" rule. If the ball lands in the zero again (so twice in a row), the bet remains "en prison". We haven't personally played on a table that offers this, but it has been reported.
Online Casinos that Offer "En Prison"
In a nutshell, we haven't found any. But that doesn't matter, as La Partage gives you the same benefit. Play Premium French Roulette at Mansion- their French games offer La Partage..
The rule does not apply to other outside bets like the dozens or columns bet. If the ball lands in zero, you lose the bet. Shame, hey?