Both the Cremona and Coinola use a mandolin rail with pieces that slide
in a unit that moves the plunger in and out of line with the hammer
shank.  The plungers differ in that the Cremona plungers look like a
little paddle and Coinola is a round piece  such as a regulating button
on a piece of wire with a little round piece that the hammer shank hits.
Both hit the string at the same time as the hammer hits the string.
When you move the hammer by hand to where it touches the string the
plunger is just short of the string. It only hits the string with the
inertia of the hammer when played.   Thin pieces of leather such as
pouch leather is used on the Cremona to make the adjustment.  

Hope this helps.
Don Teach