The En Prison rule applies to the outside bets and only when 0 or 00 pops up. Suppose you got a $10 bet on Red and zero shows. The dealer gives you the option of losing half your bet ($5 in this case) or going into prison. If you choose to go into prison, she'll put a marker on top of your bet that reads "En Prison." On the next spin this is what takes place:

1.         If zero or double zero show you lose the $10.
2.         If Black shows (you had Red) you lose the $10
3.         If Red shows, she merely removes "En Prison" marker and you can now do what you want with that $10.   Remember, it only comes into play on tables that employ 0 and 00. Technically, what it does is reduce the Vigorish against you to 2.63% by making you lose two times in a row when the 0 or 00 pops up.
In a way it confused so many people that it tended to slow the game down. So they reverted to the simplest move of all.
If you wager on the outside bets and 0 or 00 comes up, they simply take half your bet. This retains the "Vig" at 2.63 %, keeps the game going and nullifies one of the zeros against you.