Southwestern Mandarin: what does 欛新床/铺新床/压床 imply?
What needs to be understood here is that 欛新床 or 铺新床 is not just talking about making a bed or changing the sheets - but is specifically referring to Chinese wedding customs where the "new bed" is made for the newlywed couple.

Baidu 百科 has the following article: 压床（民间婚庆习俗） which explains:


  压床又称压喜床，压新床等，中国汉族婚俗之一。青年男女结婚的头天晚上，新郎家要请儿女双全的老人把床铺好，然后新郎请一个或者两个弟弟和自己一起在新床上就寝。压床的人也有条件限制，不是任何人随便就能压的。首先，压床的人必须是小于新郎的青年，一般都是自己的亲弟弟，或者是叔伯弟弟和姨表弟弟。如果在自己的亲戚范围内没有这样的人选，新郎也可以找同村的朋友和要好的同学。压床人的第二个条件，必须是未婚青年。


The night before the couple-to-be get married, the entire family is supposed to make their bed after which the groom will chose one or two of his younger brothers to have a sleep over. The persons that the groom may choose from is limited, not just anyone can spend the night with him. First, the person must be younger than the groom, usually he will pick his own fleshly younger brother or his younger male cousins. If none of his relatives are applicable, the groom may find a friend or classmate from his town. The second requirement is that the person must be unmarried.

The article also states:


  对娶亲的人来说，压床是一件吉利的事情。


This custom is thought to bring good luck to the new couple.

and don't forget:


  如果男孩儿在压床的时候，不小心尿在了新床上，这是吉利加吉利，人们更是欣喜异常。


If during this custom the boy chosen to stay with the groom accidentally pees on the bed, this is considered luck upon luck and people will be even more delighted.