Question About Fitting 12 Quavers In One Single Staff

I was watchig a video on Rachmaninoff's Piano Concerto 2 Movement 1, when suddenly this showed up:

How did he fit 12 quavers in that staff, on the bottom left? He is also using a time signature which consists of a "C" with a line in the middle, what is that one supposed to mean? Also, in the same staff, some notes have two lines coming out of them (that fourth quaver, for example), what do those two lines mean and what is their purpose?

Each quarter-note here is divided into three, with implicit triplets (they are not written, but they are there). I don't know if the triples marks appeared some measures before, but this is the reason for that.

Some notes have "two lines" because there are two notes there: the eighth-note with stems up (in triplets) and the quarter-note with stems down. It's like there are "two voices" here and they have the same note (in this case, B-natural).

He may have done this in order to emphasize these notes, for the pianist to hold them down or simply hit with more prominence, more distinction. 

I'll give you advice from my limited piano staff reading knowledge. The time signature which you refer to is known as "alla breve" (aka "cut-time"...aka 2/2). Basically it means that a bar is made up of 2 half notes (4 quarters if you want to subdivide). The twelve eighth notes you refer to are 4 triplet groups...they usually are notated as such, but if they occur multiple times during the piece, they are notated as triplets the first time they occur, and then its understood later on that they are triplets. I think Christian explains your last question well. 

Thanks guys :D i think i understood it all.

Prokofiev Piano Concerto No.2 has so much of this than is actually written in 4/4 right hand and 12/8 left hand.

lel, i'l look it up, made me curious