Complex sentence: He joined the British Army during World War I, where he passed time in the trenches by practising card manipulations, which is where he honed his ability to perform card manipulations while wearing gloves.
Split into simpler sentences: He joined the British Army during World War I, where he passed time in the trenches by practising card manipulations. As a result of many hours of practice in frigid conditions he developed the ability to perform card manipulations while wearing gloves.