In military terms, a salient is a bulge in the battlefield lines where one army finds itself surrounded on three sides by the enemy. In the case of the Ypres Salient the British, Canadian, French and Belgium armies were attempting to hold and defend the city of Ypres. The German army advanced across Belgium far enough that it surrounded the city on three sides.
About a day after a person or animal dies gases form and become trapped inside the decomposing body causing it to expand like a balloon. In water dead fish, animals and people float up to the surface belly up because of the gases. In a few days the dead fish, animal or person will sink to the bottom of a lake, ocean, river or swamp after the gases escape from their body. Here you can see that the dead soldiers' bodies have expanded because of the build up of gases.