He was comfortably backstroking in the pool, alternating his arm movements.

backstroking 是动词 backstroke 的现在分词形式。backstroke 由 back（"背部"）和 stroke（"划水动作"）组成，字面意思是"背部朝下的划水"，指游泳时仰面朝上、双臂交替向后划水的泳姿，即"仰泳"。