The effect of the block changes with the blue P-switch being on or off.

When ON, it behaves like the next Map16 block.
When OFF, it behaves like nth block after it. (i.e. block [this]+n)
(given that n is between 0 and 0xFF)
For that, enter n as the block's effect.
(note: this effect thing has something to do with the underlying
 blocktool implementation which I have no idea about since I am
 horribly out of the loop with SMW hacking)

Also, by entering 0x100+n as an effect instead, you can make its operation
depend on the silver P-switch instead.

Example 1:
200,effect(2):IF_P.blk
201:A.blk
202:B.blk

Here, block 200 behaves like
- A.blk if the blue P switch is ON
- B.blk if the blue P switch is OFF

Example 2:
200,effect(4):IF_P.blk    ;blue P-switch:OFF -> to 200+4
201,effect(102):IF_P.blk  ;silver P-switch:OFF -> to 201+2
202:A.blk
203:B.blk
204.effect(102):IF_P.blk  ;silver P-switch:OFF -> to 204+2
205:C.blk
206:D.blk

Here, behaviour is as follows:
 BLUE  SILVER
  ON    ON	A.blk
  ON    OFF	B.blk
  OFF   ON      C.blk
  OFF   OFF     D.blk

However, you perfectly may use it for less weird things too, lol~