R1   S---CH2
  \ /     \
   C      CH2  +  Ra-Ni or H2N-NH2 ----> R1-CH2-R2
  / \     /
R2   S---CH2

Some more information on that reaction can be found on http://www.tciamerica.com/news/newslib/94sum_a.htm
I don't completely see how MeS- interacts in the reaction though, how/why it leaves. I suggest there is formation of MeSH in the first step of the reaction? Anyway, as it looks related to Friedel-Crafts, I postulated the following mechanism:

           (d-)                        
  H2C---S   S--Me         F            
   /     \ /              |            
H2C       C(d+)     + (d+)B--F    ---->  
   \     / \              |             
  H2C---S   R             F             
  


            Me
            |
  H2C---S   S~~~BF3          H2C---S                F
   /     \ /                  /     \ (+)           |
H2C       C         ---->  H2C       C     +   MeS~~B--F
   \     / \                  \     / \             |
  H2C---S   R                H2C---S   R            F
  
  

  H2C---S   R               HC---CH
   /     \ /          (2)   /     \
H2C       C ----*     HC---C       CH   ---->
   \     / (+)  |      |    \     /
  H2C---S       *--->  |     C---CH
                       |    /
                      HC---N
                      (3)
                      
                      
  H2C---S   R
   /     \ /
H2C       C   H  HC---CH
   \     / \ /   /     \
  H2C---S   C---C       CH  --[MeS~BF3]-->
            |    \     /
            |     C---CH
            |    /
         (+)C---N
           /
          H
          
          
  H2C---S   R
   /     \ /
H2C       C      HC---CH
   \     / \     /     \
  H2C---S   C---C       CH  +  BF3  +  MeSH
            |    \     /
            |     C---CH
            |    /
           HC---N

Theory: in the 2-methylthio-1,3-dithiane, there is one C atom bound by 3 sulphur atoms and an R group. S drains electrons from C, by which C ends up with a partial negative current. In this case, a Lewis acid (BF3) interacts with the methylthio group. I don't know exactly why the BF3 wouldn't interact with the dithiane S atoms... stereochemical factors, or a lower partial negative current, or... ? Anyway, MeS- is the leaving group and form some kind of complex with BF3 (don't think it can form a bond, since B can only form 3 sigma bonds according to hybridisation theorem). The dithiane carbonium ion can now interact with the indole structure, as shown in the sketch. But I think the carbonium ion can interact both on C2 and C3 of the indole. I show the reaction in the way you would like it most... . The hydrogen on C2 is "cought" by the MeS-BF3 complex, by which the indole derivative becomes a stable molecule and MeS-BF3 falls appart in MeSH and BF3. This is 100% theoretical, I might be selling you a bag of elephant dung as well... but as I look at this reaction, you'll end up with two alkylated indoles: one alkylated on C2, the other on C3.

Might be interesting as well:

[1] L Micouin, A Diez, J Castells, D Lopez, M Rubiralta, JC Quirion, HP Husson - Synthetic applications of 2-(1,3-dithian-2-yl)indoles V. Asymmetric synthesis of dasycarpidone-type indole alkaloids, Tetrahedron Lett 36 1693-1696.
[2] Seung-Hwa Baek, Chan-Nam Yook, and No-Yeun Park - Boron Trifluoride Etherate-Catalyzed Formation of 3,4,5,6-Tetrahydro-7-hydroxy-2-(1,3-dithian-2-yl)-9-alkyl-2,6-methano- 2H-1-benzoxocin Derivatives, Bulletin of the Korean Chemical Society 12(6) (1991) 633-635. http://www.kcsnet.or.kr/publi/bul/bu91n6/633.pdf

PS: pi-bond in sketches not shown due to estetical considerations... you (should) know where they are .

  H   S---CH2
   \ /     \
    C      CH2 
   / \     /
MeS   S---CH2

and the yielding indole-1,3-dithiane is hydrolyzed with Hg(II) (for instance HgCl2) instead of reduced with Ra-Ni, the end product is an aldehyde.