King-six doesn't look like much, but with a stack this short the push/fold chart says to move all-in, and it shoves this hand with up to 15 big blinds. Moving all-in also means the big blind can't raise you off the hand.
You collect the 900 in the pot. That's your own 300 back plus 600 from the big blind, which is what a shove earns when it isn't called.