The Super-Block Meter is a gameplay mechanic of Plants vs. Zombies Heroes. At every attack, the Super-Block Meter charges, and when fully charged the attack is blocked and the player receives one random superpower from their hero's superpower kit. The superpower can be used immediately for free, or turned into a 1 or 1 (depending on the side) card to be used at a later turn. Super-Block Meter will only work properly if there is space on the player's hand for the Superpower card; if the player has 10 cards or more in their hand and their Super-Block Meter is full, the meter will fail to block attacks until the player dispose some of their cards.
After every hit, there is an equal chance of charging one, two or three bars of the block meter out of 8 bars. The meter can only block three times before being destroyed, and the only way to attack the heroes without raising the meter is via the Bullseye ability.
Even though it is based on odds, it is possible to turn the Super-Block Meter in your favor. If there is a low attack minion on the field like a Imp or Peashooter, it is usually preferable let them attack you in order to gain Superpowers at a minimal health cost and to prevent the opposing hero from replacing those weak troops with stronger ones. Also if there is one very powerful troop and the meter is almost full, let it attack so you can survive another turn, which may be the key to victory. Grapes of Wrath's ability can easily be avoided by timing the Super-Block Meter correctly.
The best ways to counter this mechanic is with the Bullseye ability, which is available on both sides, although on the plant side this is not very useful as the plants with this ability have very low strength points. If you see this ability, don't let the troop attack the hero, as there is no positive outcomes out of it. Another counter is the Shieldcrusher Viking, which will drain the entire meter with each direct attack.
—Devin Low, Creative Director and Lead Designer, Plants vs. Zombies Heroes
Insta-Block (called Mega Block prior to version 1.4.14) was a mechanic that existed in older versions of Plants vs. Zombies Heroes. In those versions, the player had a 5% chance of receiving an Insta-Block whenever struck by a non-Bullseye attack, which would fill the block meter immediately regardless of current charge. It was removed on update 1.6.27, as PopCap decided that the mechanic introduced an unnecessary layer of luck to the game and created too many unfair situations.