Ok here is how this works- clan leader donates a gold, elixir, or gem amount for a certain achievement by a clan member, first to that achievement gets the loot. The clan leader gets the loot to be awarded from Supercell every week as an allotment, and if the clan leader does not initiate the contest the loot is lost. Maybe Supercell also gives the leader a choice of three contests and they pick the one they will run in their clan.
For example- maybe the clan leader wants to see general improve net in the clan- so the wager it five gems for the first clan member to earn 100 trophies. So the contest goes up, clan members start working for it, first to get it gets those five gems.
Edited to account for a modification that would stop donating of resources by clan members.




