monSysWaits event 150 - what a meaning of "Number of times tasks waited..."

In case of monSysWaits event 150 - how can I interpret the waits "Number of times tasks waited...".

Is this always the number  of blocked processes?

And why the counter changes only after block has released, but not during the block.

Actually when block started the system already waits, but the monSysWaits doesn't indicate this.