It is actually not that straightforward to know what to buy when the whole market is down significantly. In 2008/09 I thought I was really clever buying an ETF over the ASX200 at what turned out to be very close to the bottom in March 09 (about 3200) but that was actually the wrong decision: apart from ETFs often lagging the index they track (due to ETF holdings not exactly mirroring the underlying and more in built costs than you might expect which compound the lag over time) I found to my chagrin and surprise that a year or two later I had only made a 20-30% return. This is after one of the greatest bear markets in history.
On reflection, I realised that what I should have done is - rather than look for broad exposure to the market or even robust companies that will plod away through thick and thin - buy deep cyclicals. The reason for this is because, provided they make it through (a big if, but the thinking is that the majority of those that have failed will have failed by the time of a 30-40% market pullback) then they will enjoy far greater margin and profitability expansion than the companies that have been profitable throughout.
This is simple maths: to go from 1 (or even -5) to 10 is a much percentage increase than to go from 10 to 20. Multiple expansion often compounds this increase.
So buy Flight Centre (from $4 to $20 in a year and at one point $60) or mining contractors or service providers rather than supermarkets or non-cyclicals (which is what you should be holding - although of course cash would be better - prior to the 30-40% broad market fall).
This is not to say I am not a fan of dollar cost averaging into ETFs as an entirely legitimate and profitable exercise for almost all investors, it is just not the correct play at market bottom.