Addressing this:
How?
The Void is the Sentry's other, his negative equal. For every good action the Sentry does, the Void counters negatively (e.g. Sentry saves a boat with 123 people, Void causes airplane crash killing 123 people). In the 2004 miniseries, when the Void struck first, it allowed the Sentry to "balance the equation", allowing Sentry to throw the Void in the sun.
Keep in mind, however, that this is temporary -- the Void will return, as noted by Sentry. So as long as the Sentry exists, so will the Void. Heck, with all the good Sentry has done in the Void's absence, it's probable that the Void is tallying it up and will eventually counter it big-time when he decides to return.
If the Sentry faced the Void like he did Hulk in WWH#5, the Void, being Sentry's other and equal, would probably adjust himself accordingly, "countering" Sentry, so to speak.