'strict' == strictly what you put there will be used by the DNP or whitelist

unchecking 'strict' will stop all forms of the key you press.
ie:
if you put W in the list and 'strict' is unchecked, shift+W, ctrl+W, and alt+W will also be blocked/allowed (depending if you are using DNP or whitelist)

the purpose is to reduce/eliminate the potential people have for run-aways.

run-aways can occur while using 'strict' due to modified key sequences and keys being released in a certain order.
ie:
let's say you have W on your DNP
then you hit these keys:

shift down
W down
shift up
W up

what will pass to the other wows is:

shift down
W down (since this is shift+W.. which is NOT on the DNP)
shift up

and that's it. the 'W up' was not sent since W is on the DNP

and due to how blizzard does their movement model, down means go.. up means stop.
you may have W bound to a key, and shift+W not bound... blizzard will interpret any W as a W .. and will move you forward.
since the W up never came in... your other wows just keep moving.

if you use strict, you could just add all the modifier keys with your movement keys, and you'd be covered.
ie: shift+WASD, ctrl+WASD, alt+WASD