Summary
Item 42 and item 42's quantity control wMapScriptPtr (D36E-F in Pokémon Red and Blue and D36D-E in Pokémon Yellow), with the index number of item 42 being the first byte to a little-endian pointer, and item 42's quantity as the second. This word contains the current map script (not to be confused with the meta-map script which is not controlled by wMapScriptPtr).This script is run continuously after the menu is closed. The address can be changed to one corresponding to a different item slot, such as Water Stone x211 (Thunderstone x211 in Yellow) to make the script point to item 3 (D322/D321).
This is an efficient way of arbitrary code execution, but the items in slot 42 will be wiped after leaving the map, so it may be a good idea to swap the original map script back in before moving to a new map.