Which reaction is this? In my book there is a reaction that is


  $\ce{HCCl2-CH2F}$ in presence of $\ce{RO^- ROH}$ gives $\ce{CCl2=CH2}$


I could not understand my type of reaction it is?

And why does $\ce{F-}$ leave whe $\ce{Cl-}$ should leave as it is a better leaving group?
It appears to be an e1cb mechanism, where formation of the carbanion on the dichloro end is the rate limiting step. This appears to be a far more acidic proton than the ones on the fluorine-bearing carbon. Once the carbanion is formed, it rapidly eliminates fluoride ion to give the stable dichloroethylene product, rather than eliminating chloride (admittedly a better leaving group) to give the carbene, a higher energy product.