Chris Kosednar of Carleton College's CUT ('09 college champs, perennial contenders and manufacturers of some of the best club players in existence) has this to say about one's defensive attitude when covering handlers.
PRIORITY: Don't get beat upline. After that:
--Deny the swing
--Be conscious of the dump (be able to contest a poor dump pass)
--If you can't make a play on the dump pass, cut off the around swing (that's a backhand break in this scenario)
--Next, deny the inside swing (inside flick break)
--From there, you should be in a normal mark position at a 2-3 count
(For the above scenario, it's best to imagine that the defense is forcing flick and that everybody on the field is right-handed. Sorry Falkor.)
Above is a good example of an uncontestable dump pass. The defender identifies the break opportunity and adjusts to prevent it.
Why is this advantageous to the defense?
It prevents the opposing handlers from using the entirety of the field to implement their offense, thereby limiting their offensive options. It also moves them backwards and limits the lane size for cutters to operate within.