We covered a lot of this about a year ago under a thread called "commercial checkride" in the Flight Training section. Rick was part of that conversation and I agree with his points.
Regional Airlines want some number of hours of legal multi time. It doesn't necessarily need to be PIC, but most of the time will likely be PIC because of the nature of trying to get it with low hours.
As brief as possible, here are the regs related to it:
14 CFR Part 1.1:
Pilot in Command means the person who: (1) has the
final authority and responsibility for the operation and safety of the flight; (2) has been
designated as pilot in command before or during the flight; (3) holds the appropriate category, class, and
type rating for the conduct of the flight.
When an airline says "PIC" in their min requirements, this is the definition to which they refer. They consider the "designated pilot in command" as the pilot who signs for the aircraft or who has been told by the company "you're in charge".
14 CFR Part 1.1:
Second in Command means a pilot who is designated to be second in command of an aircraft during flight time.
Designated means required by the company because they are operating under FAR's and/or Op Specs that require a SIC, or because the aircraft type certificate requires a SIC.
14 CFR Part 61.51(c)
Logging of pilot time. The pilot time described in this section (
meaning part 61.51) may be used to: (1) apply for a certificate or rating issued under this part (
meaning part 61); or (2) satisfy the recent flight experience requirements of this part (
again part 61). What this means is the regs for logging PIC time in part 61.51(e)
only apply to logging time for certificates and ratings or currency.
Now for the infamous 14 CFR part 61.51(e)
Logging pilot-in-command flight time. (1) A recreational, private, or commercial pilot may log pilot-in-command time only for that flight time during which that person - (i) is the sole manipulator of the controls of an aircraft for which the pilot is
rated; (ii) is the sole occupant of the aircraft; or (iii) except for a recreational pilot, is acting as pilot in command of an aircraft on which more than one pilot is required under the type certification of the aircraft
or the regulations under which the flight is conducted.
Ok, a couple of notes. In (1) notice it leaves out airline transport pilots. Once you get an ATP you can no longer log sole manipulator time. Also in (1) "rated" means category, class, and type, if the aircraft requires a type rating. Note it also says "flight time". This means pilot time which occured in "flight". Simulator (regardless of level) or FTD time is not flight time and does not count.
If you are a multi-engine rated pilot (commercial or private with the appropriate type rating if required by the aircraft) you can log "PIC" time while you are sole manipulator flying the aircraft
for the purposes of meeting the requirements of a certificate or rating. This is legal multi time, and as long as the airline doesn't require Part 1.1 PIC time, it meets their mins. In almost all cases this is only for flights under part 91 - not for flights under part 125, 135, or 121.
Notice the last line "...or the regulations under which the flight is conducted." This also includes 14 CFR Part 91.109(b)(1) - the "safety pilot" rule. Yes you can be a safety pilot and log PIC flight time as long as you log
only the time the other pilot is under the hood, not the whole flight.
Lastly - MEI's. 14 CFR Part 61.51(e)(3) An authorized instructor may log as pilot-in-command time all flight time while acting as an authorized instructor. The key word here is "authorized". The FAA and NTSB have come down very hard in the last couple of years on the time honored technique of getting an MEI, finding someone on the airport with a twin and getting the owner to let you ride right seat while you "call it dual". A legitimate instructional activity must be occurring. Several MEI's have lost their tickets recently for doing trying to build time this way.
So ways to build legal multi time - get a multi rating, get your instrument rating in a multi, get your MEI and teach, get your CFII in a multi (a great way to save money is to train for your CFII in a multi, after you get your multi rating, because you need 15 hours of multi PIC time before you can take your MEI ride - might as well do your CFII training while you burn off those 15 hours), fly as a safety pilot for a fellow multi pilot (just log it correctly), get lucky and find a legitimate SIC job in an multi aircraft where the operation or FAR's require a SIC, or if you have the funds, by a multi, fly it, then sell it. However, DO NOT buy a multi (or rent one for that matter) and offer to take people flying if they will pick up part of the cost - CAREFUL - you may be holding out and will quickly fall under additional requirements of Part 119 (which leads to Part 135 etc.). (see the thread "Commercial Checkride" in the Flight Training section).
I guess that wasn't very brief was it.