|Vaali and Sugreeva, chase a rakshasa to kill him, but the rakshasa enters a den and so Vaali enters the den and asks Sugreeva to stand guard. Vaali never returns even after a year and finally Sugreeva sees blood oozing out of the den. Sugreeva thinks that Vaali is dead and so he closes the den with boulders so that rakshasa never comes out and returns back to Khiskinda.
Later, Vaali comes out of the den and misunderstands that Sugreeva has taken undue advantage of the situation, so he abducts Sugreeva's wife and sends out Sugreeva on exile.
When Sugreeva meets Rama, he gets into a pact where in if Rama kills Vaali, then Sugreeva and his Vanara sena can help Rama in fighting Ravana and getting back Seeta.
Vaali, has this unique boon where in if anyone who fights face to face with him, half of the opponents strength will be transferred to Vaali. Hence, Rama knowing this decides to shoot him from the back.
But it is very difficult to identify who is Vaali and who is Sugreeva on the first day of fight, hence, they come up with idea of garlanding Vaali intial saying that he will win.This garland helps Rama identify Vaali and kill him standing behind him.
In the above scene, Rama is seen standing behind Vaali, you can find Vaali with a big garland and Rama is sitting in posture leaving the arrow towards Vaali.