He sees the sigil a moment before Gabriel's hand hits the wall and only has time to take one step forward before the connection. He doesn't know what could possibly happen-- where the sigil would banish them to, or whether it would toss them both into the Rift, or if it would even work at all.

There are a few tense seconds where nothing happens at all. There is just Gabriel, tense and intent, his hand pressed against the wall, and Lucifer, in an aborted movement to stop him.

When it's clear that the sigil has failed, Lucifer takes those few more steps over to his brother and pulls his bloody hand from the wall, grip firm around his wrist.

"Gabriel."

His voice is cold; this was foolish and possibly suicidal, and he doesn't appreciate his brother gambling with both their lives.

"What did you think this would do, Gabriel?"