When the price of eth is crashing, some people will be fleeing eth for dai but some existing dai holders will be scooping up eth on the cheap. So the net effect here isn’t clear. Now since CDPs will be gradually undercapitalizing, some CDP holders will be either dumping eth or DAI into their CDPs, depending on which they have, to shore up their CDPs and not lose all their collateral. The circulating supply of both eth and dai will decrease from being locked up in CDPs (increasing the dollar price of both), offsetting the sudden undercapitalization, somewhat.
As I understand it, the TRFM doesn’t kick in for an event like this. Instead, in the short term, PETH will be diluted to help capitalization. TRFM only concerns the market price of dai. If the net effect on an eth crash is to cause a rise in dai, then then TRFM will be negative. If the net effect is a fall then the TRFM will be positive.
The net effect on the price of dai from a sudden eth crash will really depend on the relative holdings of the market participants and their risk profiles.
BTW Dai has survived quite a severe eth crash without triggering any emergency measures.