Try pasting one of these into a city save.
For instance, NTCT and PTCT naturally cool themselves. While sparked, they still do this, but it's noticeably slower. And WATR and MERC will not move while sparked. Basically, anything that is currently sparked becomes an unmeltable, immobile, unbreakable solid while losing its normal properties until the spark passes.
The reason WHY it allows pressure is that any conductor, while sparked, changes its element type to SPRK with a ctype of whatever it was before sparking. In the case of TTAN specifically, this means that the game no longer recognises it as TTAN, so pressure passes as normal. Any other conductors with unusual properties are also likely to lose them while sparked.
A better way of expressing the key point here would be, "Sparked TTAN doesn't block pressure". This is already known to experienced users, but isn't immediately obvious to newcomers.