Refers to ease of realising that the rangefinder has "found" the correct focus distance.
Dim rangefinders using less expensive optical arrangements, so are harder to see this snap.
Leica seems to be good. Although many have complained of a problem in some newer Leica M rangefinder models. They say there is flare. Very odd as you pay $3,000+ for the body.
You don't expect this poor performance after paying so much.
Have seen one knowledgeable buyer of Leica M6 insisting on buying the older (1980 to 1990) made Leica M6s in second hand shop and very willing to pay EXTRA. He refused to buy the newer Leica M rangefinders. Apparently some cost cutting measures and internal parts use cheaper materials. Not so sure about this issue. Anyway I had 3 in the past and these were the 1980's variety so I experienced no problems at all.
I have an old Shanghai rangefinder that cost very little but has snap.
In those days, they did not stinge on materials. Could have been made in the 1960s or early 70's. Nowadays cost cutting equals more profits equals higher bonuses for staff.
Once in a while you get a piece of BRAND NEW lens that has problems (psst...go to Canon thread and read messages on the latest L lens).