In "Qpid," Riker and the rest of the senior staff go in search of the captain and Vash, when they find that Picard's execution by beheading is imminent. Riker, noting the number of armed men surrounding them, tells Data that the team needs a diversion. Data then removes a component from the inside of his arm and tosses into a nearby fire, causing a large explosion. Is there any information anywhere what this component was? Obviously it was either unnecessary or replaceable, but I'm not sure if its function was ever stated.
The script shows that a line was cut from the finished episode (whether or not it was filmed) explaining that Data removed some "microfusion cells":
RIKER : Data, we need a diversion. Now.
Data opens his left arm, revealing the circuitry and blinking lights within. He pulls out three small components and fastens them together.
DATA (to Riker) : Please stand back, Commander. Microfusion cells can be somewhat dangerous under high temperatures...
Riker stands back. Data tosses the components into the Tinker's fire. The fire FLARES briefly, then there is a small explosion and a dramatic pyrotechnic discharge which startles the Executioner and the onlookers.