The petard depicted in this model was a shaped charge/bomb used to demolish gates or walls. Perhaps the word is best known today from Shakespeare famous phrase "hoist with his own petard" taken from Hamlet. This device must have been familiar, and probably infamous, enough to the Elizabethan crowd to be mentioned into the play and to have understood how dangerous it was.
During the English Civil Wars a petard was successfully used at the siege of Farnham Castle in November 1642 and unsuccessfully at the storming of Bristol in July 1643.The first figure is shown sensibly wearing a helmet to help deflect the shots of alert sentries.
Although there are no specific rules for petard teams in the forthcoming Pikeman's Lament they can form part of a Forlorn Hope unit, an elite group that are used in the game to hold or assault positions.