I understand that depth jump is vital in developing reactivity and explosive power, and that they should not be used all year. If so, when should i add it into my plyo exercises? My training phases are like this:
Off-Season (June - Sep)
Pre-Season (Sep - Feb)
In-Season (Feb - May)
multiple factors are involved
what are your events?
how experienced are you?..how strong are you?
have you done them?
as a jumper i do them year round
as a sprinter i would do them preseason
Oni,
try and combine it with your top speed focus, but is this for your 400 m?
This is what I think of Depth Jumps.
What’s your reasoning in combining depth jumps with top speed focus?
Sorry, I wasn’t clear on a couple of points.
Depth jumps followed by hurdle hops of similar nature (i.e., vertical). Jumps of low height, of course!
I “wouldn’t pay” much attention to them for a 400 m runner preferring those of a more “flat” nature and before appropriate sessions and of right reps (that’s why I asked Oni).
I wouldn’t use Herb’s post as an example 
Lastly, for the vertical nature involved in top speed.
PS It might be the last stage to include, if at all, as simple hurdle hops may suffice and of course, depending on the individual and issues involved with him/her (e.g., background on plyos), as the risk involved might not worth the effort.
Did you have anything specific in your mind for asking this? 
Let us know of what you think!
Well I know that depth jumps might not be beneficial for a 400m runner, i am also a long jumper as well. So i feel the need of incorporating that into my workout.