EUR/HKD represents the euro priced in Hong Kong dollars, that is, how many Hong Kong dollars one euro will buy. Because the Hong Kong dollar is pegged to the US dollar, this cross behaves in an unusual way: it largely mirrors EUR/USD, simply scaled up by the peg. Liquidity is reasonable but second-tier, with spreads wider than a benchmark major.
The euro is the base currency and the Hong Kong dollar the quote currency, so a quote of 8.4500 means one euro buys 8.4500 Hong Kong dollars. A rising quote means the euro is appreciating against the Hong Kong dollar, a falling one the opposite. You take part through a forex CFD, going long if you expect the euro to climb and short if you expect it to drop, rather than exchanging the currencies themselves. A pip is the fourth decimal place, and your result is pips multiplied by position size.
What really drives this pair is the euro leg, because the Hong Kong Monetary Authority holds the Hong Kong dollar inside a narrow 7.75 to 7.85 band against the US dollar under the Linked Exchange Rate System. With the Hong Kong dollar barely moving, EUR/HKD effectively tracks EUR/USD scaled by the peg, so European Central Bank policy and the dollar's wider swings dominate, while the HKMA's defence of the band keeps the Hong Kong side anchored.
Say EUR/HKD is trading at 8.4500 and you buy one standard lot (100,000 euros), expecting the euro to advance. Each pip is worth 10 Hong Kong dollars, so a 40-pip rise to 8.4540 gives:
40 √ó 10 = 400 Hong Kong dollars (about $51)
A 40-pip fall to 8.4460 would set you back the same 400 Hong Kong dollars. Since the position is leveraged, you commit only part of the 845,000 HKD contract value as margin, amplifying both the upside and the downside.