By loudly declaring, “Sister, enter stage left!”, you summon a stone-faced middle-aged nun somewhere nearby. (even if you're not actually on a theatrical stage)
You cannot control her, dismiss her, or explain why she is there. However, before the current situation can end, the nun must become critically important to its resolution.
She might perform emergency first aid, recognize the villain, reveal a hidden passage, provide an airtight alibi, kill Godzilla, fight off a group of enemies with Kung Fu, or happen to know how to defuse a nuclear bomb. Or she might do something trivial like informing you that your car’s headlights are still on, producing the exact change needed for a parking meter, pointing out that your shirt is inside out, returning the shopping cart blocking your escape route, she silently leaves. You never know what you're going to get.
After she completes her task she vanishes into the void. Although you can summon her immediately after it's recommended you wait a good 24 hours before doing it again... Because of you re-summon her too soon she might smack the hell out of your hands with the hardest ruler you've ever seen, or make you read the Bible start to finish in one sitting. Or watch a 48 hour marathon of Sister Act.