When a player dies, their team has a chance (default 100%) to lose a random technology. Technologies currently being used as prerequisites for other fully researched techs are immune to this loss, and options exist to bias the random selection based on tech age or cost.
Any machines using recipes lost this way will be suspended until re-acquired, though any items already acquired are kept (just as though you'd found them in a goodie box), you just can't make new ones. This behavior can be turned off as an option.
Some mitigation options are available, from friendly fire protection to periods of immunity for new players or newly respawned players.
After "unresearching" a technology, the mod tells Factorio to recalculate recipe access and bonuses, so there may be compatibility issues with mods or scenarios that make manual changes to a force (team)'s bonus.