Valve Crossover.....
 

...pointers to what, when, why, where and how please......

Suspect it has something to do with ICA and both valves being open at the same time, but clarification would be nice.

Thanks chaps/chapesses

on what the question is about please?

basically both valves open earlier and shut later than BDC and TDC.
the way bob tait explains it doesn't really sound right but if you can accept that the gas has a "mass" and therefore "inertia" keeps it exiting/entering the cylinder after the piston has reached TDC/BDC
hope that kinda helps:ok:

Thanks Swanie....

Thought that's what it was but never heard it refered to as 'cross-over'.

'Lead', 'lag' and 'overlap' are the more common terms re valve timing design, DD. You might find this interesting:
http://www.compcams.com/Technical/TimingTutorial/