If you do, put a +1/+1 counter on it
If the chosen card is cast as a spell, you control the player while that spell is resolving
Like Card Rush, you look through the showcases of cards, then call a staff member to pick out your order for you

Aesi, Tyrant of Gyre Strait Benthic Anomaly Benthic Behemoth Biolume Egg Biolume Serpent Bladecoil Serpent Bog Serpent Caetus, Sea Tyrant of Segovia Cryptic Serpent Deep-Sea Serpent Deep-Sea Terror Denizen of the Deep Dirgur Nemesis Dreamwinder Floodtide Serpent Frilled Sea Serpent Gearseeker Serpent Harbor Serpent Iceridge Serpent Junk Winder Koma, Cosmos Serpent Koma, Cosmos Serpent Koma, World-Eater Kukemssa Serpent Lava Serpent Lightning Serpent Loch Mare Lochmere Serpent Marjhan River Serpent Ronom Serpent Sandbar Serpent Scrapdiver Serpent Sea Monster Sea Serpent Serpent of Yawning Depths Serpent of the Endless Sea Serpent of the Pass Shoal Serpent Shrouded Serpent Slinking Serpent Slipstream Serpent Sliptide Serpent Spire Serpent Steelclad Serpent Striped Riverwinder Sword Coast Serpent Taniwha The Terror of Serpent's Pass The Unagi of Kyoshi Island Tolarian Serpent Tolarian Terror Verazol, the Split Current Vodalian Serpent Wormhole Serpent Xolatoyac, the Smiling Flood Yorion, Sky Nomad

I've watched players agonize over whether they have enough damage modifiers to secure a knockout, all hinging on these plus symbols stacking correctly