When a tonk fires, you take your pool of dice, which is usually 4 for medium tanks. You roll and hope to see hits (the penetration symbol) and crits (the WoT symbol). Defense pool is actually built - one for armor, one for each time the target and the shooter moved, etc. - but can never go above six. It can be made smaller by flanking and shooting point blank. Rolling hit symbols allows the enemy to choose which hits are cancelled, while crits leave the choice to you.
