Depending on what company you work for, it can be a life saving piece of contract language, or not worth the paper it's printed on.

The GENERAL language goes something like this; If you commute, there has to be TWO flights to get you to domicile before showtime. If for some reason you get bumped off, or it cancels due to WX or MX, you call scheduling to inform them that you tried to get to work but were unable to make it due to circumstances out of your control. You will get pulled off the trip and a reserve pilot (or a reassigned line holder) will get your trip. And yes, you lose the pay credit. Again, depending you my be allowed to join your trip when you make it to base and fly (get paid) for the remainder, or you may be out of luck and hope there is some open flying to pick up to recoup the loss of pay.

Like I said, the above is the GENERAL language of most commuter clauses.

There can a lot of caveats or "gotchas" written into them, you simply have to read the contract and follow the rules to use it. An example can be that you can only use it so many times in a year, or you have to prove that there were seats "available for sale to the public" when you listed for the flight, if applicable. Another can be that if you were going try to commute tomorrow and a blizzard is forecast, you may not be able to use it because of exercising "bad judgement" trying to commute in those conditions.

In some cases, some guys have their own "version" of a commuter clause that ensures they get paid.