What causes word blocks to light up in Baba is You?

While playing Baba is You, when I push a word block (such as PUSH or OPEN) into an active position (where it forms part of an active rule like "FLAG IS OPEN"), it sometimes lights up: the color of the block becomes brighter. However, this doesn't always happen. In the gif below, you can see that FLAG IS PUSH is active (since I pushing the flag), but PUSH is not lit up. However, once OPEN is pushed in, making the rule "FLAG IS OPEN", the OPEN block does light up!

A gif alternating between showing PUSH and showing OPEN in an active rule with "FLAG IS". It appears to show that OPEN lights up while in that active position, but PUSH does not.