The Explorer and Monk have a unique mechanic; in the event that they fall to 0 hitpoints, they will collapse in pain. Instead of recovering them via sending rescue units, the player has the choice to pay 100 coin as Ransom to whoever dealt the final blow (including Mother Nature).
Essentially, this technology is used to restore a fallen hero back to Town Center by paying 100 coin. This can also be used to recover the Explorer's Dog, who (unlike Spanish War Dogs) falls unconscious rather than dies.
It is not always a good idea to choose to Ransom a hero, as the 100 coin invested go to the player who dealt the fatal blow. This is especially the case in the early-game, when resources are harder to collect, and a sudden boost of 100 coin can make the difference.