I keep hearing this saying but i dont quite understand why it happens ? Could someone please enlighten me on this ?

Thanks Guys
Richard

 

There are several reasons a valve could 'drop'.

It basically refers to a valve hitting the piston for any number of reasons. The valve keepers could fall out and cause the spring to come out. The valve spring could break. The engine could be revved too high and the rocker lofted over the nose of the camshaft. The valve stem could break or the head of the valve could break off. The timing belt could break or the cam could have insufficient piston to valve clearance.

It's not a specific condition but describes a number of different causes.

 

dropped valve:

this one came about from valve float