So imagine you're playing a low stakes game of ultimate. A regular team training or a weekly league game.
Here are two scenarios:
1. You call match ups, assigning a teammate to defend a particular opponent
2. You say "match up across".
Always do scenario 1.
Call match ups.
If you simply just guard the person across, it means the line caller has invested almost no focus into this point. It means who you guard doesn't matter. It means how you play this point doesn't matter. It means throwaways are fine and jogging on defence is fine.
And that leads to sucky ultimate.
So always call match ups, even if the game is low stakes. It will make the ultimate better.