This just means actually casting a spell, which is reflected in the oracle text Whenever a player casts a spell, Spellshock deals 2 damage to that player. Rug of Smothering works exactly as expected
As a competitive spike, I find strategic risks like this interesting and rewarding
MTG Arena is hosting a bevy of events across a variety of formats that put an emphasis on Magic: The Gathering FINAL FANTASY
If you need a playset of a specific rare for a Standard deck, buying four singles costs less than gambling on Play Boosters where you might open zero copies across a dozen packs
A second wave of similar tri-lands showed up in Khans of Tarkir, such as Nomad Outpost (taps for red, white, or black)
One mana for a Shock is completely fine, and scaling up to four damage later is even better