Yes N.G. explained it well
say something .. ok
I don't use pure Sphagnum often. However, Sphagnum has the remarkable ability to hold a huge amount of water while simultaneously holding a huge amount of air. I compress it just enough to give the plant some stability. A flopping around plant = damaged plant. I try to maintain the Sphagnums ability to hold air and moisture. The more you pack it the less air it will hold. It is also possible to pack it so tight you reduce the water holding capacity too!
Is that a Nepenthes in your pocket or you just happy to see me?