How do the progressive jackpots work?

[Simple Sam]

I've looked at a couple Bally progressives (889 & 952) but they have both been broken so I couldn't see how the progressive jackpot worked.  I saw this in the Bally 1100 manual:

The Progressive Feature has two alternately lit & advancing Jackpot Counter Systems building up as
the game is being played. When this jackpot is hit,the game locks out, bell is sounded & jackpot indicated by lit arrow is entirely attendant paid. This
Jackpot Counter System must now be key reset and
the Lockout System key released.

I couldn't find out what actually happens when a jackpot is won.  What does "the game locks out" mean?  Is it not playable at this point?  What is the Jackpot Counter System; is that the key on the side of the cabinet?  What is the lockout system key that needs to be released?  Is the jackpot reset to $0 or some other value?
Thanks!

[DavidLee]

This progressive machine adds a penny for every dollar played.
Jackpot alternates every forth coin.
If you win one or the other jackpot, the machine shuts down and money is paid by an attendant.

[Simple Sam]

I screwed up my original post, it's corrected now and has the details and actual question I was asking.

[slcjeeper]

Each set of Jackpot reels will have their own reset lock on the right of the machine. By turning the key, the reels will move 1 click. Holding the key turned will not drive the reels back to zero. Keep clicking the key until the jackpot amount is at zero or wherever you desire.  These keyed switches are different than the Jackpot lockup reset on the lower right side of the machine.  Upon a Jackpot win causing the machine to lockup, turn the key and the machine will accept a credit and you can play the Jackpot off.

[DavidLee]

View of reset key locks, but as in most cases no keys.

[Simple Sam]

It looks like the keys for the jackpot reset keys in the upper cabinet are a 785 only thing.  The pictures I've seen of 889 and 952 only show a key on the right side of the lower cabinet. There's a reference to the 785 JP reset switch (but not 889/952) in the Bally 2400 manual on pg 31 http://bingo.cdyn.com/slots/bally_manuals/bally%202400%20-%203%20reel%20slots.pdf

[DavidLee]

Possibly the reset switch/buttons could have been inside the machine.
Just a guess, but makes sense.