Hopefully a thought experiment will help you understand how a tighter tramp increases stiffness. Imagine that a catamaran was molded in one piece out of concrete, including the "tramp" between the hulls; or carved out of a solid block of wood; or laminated out of carbon fiber in one piece. Pretty easy to imagine how stiff any of those structures would be, right? The concrete boat would be a so heavy that all you could use it for was a neat reef, the wooden boat would be lighter but still heavy and need waterproofing, the carbon fiber boat might be fantastic but VERY expensive. Because our tramps are made of flexible cloth, they introduce flex into the total structure. The looser they are, the more flex there is and the tighter they are, the less flex there is. Does that add any clarity? BTW, Some beach cats actually have a solid "tramp", like the wooden or glass Shark catamaran.