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human
[Infinity War] (small spoiler) Why did Doctor Strange's sling portal...

During the fight between Ebony Maw and Doctor Strange, Maw "shapes" some bricks into points and launches them at Strange. Strange opens a sling portal in front of him to intercept the points. He then looks at Wong who opens another sling portal in front of him and the points fly out back at Maw.

When a sling portal is opened, it's opened in two places at once (at the entrance and exit). When Strange opened the portal that intercepted the points, the other end of that portal should have automatically opened at the same time — and would have probably best been opened above Maw so that he wouldn't see it coming.

So why was Wong needed to open a second portal? And where were those points in the second or two between when Strange's portal took them in and Wong's portal spit them out?

**EDIT:** Solved, unless a better explanation comes along.
gpt
The bricks probably entered the same place that Loki went when Strange dropped him and he fell for "30 minutes". They were plummeting for the second or two.  Wong and Strange set this up as a contingency for combat situations like this.

Wong had to locate the projectiles in the falling dimension or estimate their position and then open a portal in front of them with an exit point aimed at Maw.  Ideally he'd put it right next to his target so there would be no time to react, but it's a complex maneuver, so just getting them going in the right direction was a victory.