Saturation Seal

Hard Used by Smart Hint

Filling this connection all the way would leave both its islands full, and with them full nothing else could ever leave their group — so it would strand the highlighted islands from the rest of the board. It can't carry that many.

Existing bridge
Connection ruled out
Islands that establish the pattern

How it works

The 2 on the left edge already has a bridge up to the 1 above it, so those two islands are one group — and that 1 is now full.
Sending the 2's remaining bridge straight down to the 1 below would fill both of them at once.
That would leave every island in the group full, so no bridge could ever leave it again — the group would be stranded from the two islands on the right, and the board has to end up one connected network.
So that connection is impossible: the 2's last bridge has to go right instead. Two 1-islands, or two 2-islands facing a double bridge, are the simplest version of this same trap.