Well, there's a lot of questions in that short paragraph.
Rear end coming around at speed:
The first thing to address is proper suspension settings. Given all of the posts on this site relating to suspension, I'll assume you have properly tuned your suspension. If not, read no further, as your speed will always be restricted by improper suspension tuning. I suspect you may still need a little work in this area.
A Scotts dampener is also a huge plus, as it helps control the movement of the bike at the steering stem. A great investment.
After suspension is properly tuned, riding position is the next item on the list. There is a "Sweet Spot" on your bike in the standing position as you move your weight from the rear of the bike to the front of the bike. When standing in this sweet spot, the bike's suspension is allowed to operate properly, and quite frankly, you don't even feel the whoops...really (this is how to conserve energy. After mastering riding position, I can ride several hundred miles in a weekend of racing, and never get a blister). Most folks ride with their weight too far to the rear of the bike, which restricts the action of the rear suspension. I have a picture in the gallery of the proper riding position. Take a look. You will notice that the arms are almost pointing staight towards the ground which means the rider has his head over the front of the bike, and the bars are in line with his nipples (can I say that here?). In this correct position, you should be able to see your front number plate.
Technique
There are actually two different techniques to go through sections of whoops. The first technique is to grab a handful of throttle and pound the tops of the whoops, keeping the front wheel light so it does not drop into a whoop valley (not sure how anyone is able to make this happen on a 125??). You see this technique used in supercross quite frequently. The idea here is to let the front and rear wheels hit only the tops of the whoops in an alternating pattern. As you might imagine, this takes speed, and certainly committment. Lose your nerve in the middle, and you're likely to land on your head.
The second technique is timing the whoops and making them into small jumps that you double and tripple. This technique works well in short sections, or as you enter a whoop sectoin, but long whoops are often not spaced well enough to make this technique effective for long sections. My personal oppinion is that this is a slower technique, but have met folks who disagree. Funny, they are never able to keep up in long whoop sections for very long.
In reality, a combination of these two techniques is usually employed.
On short sections of whoops, assuming the first whoop is large enough, I will try and jump as far into the whoop section as possible (I do this as well if I am transitioning through a ditch in a long whoop section. Some risk here, as you have no idea what lay ahead, and I have landed in a few rock piles. Never crashed as a result, but sure made for a little excitement). The next step depends upon what the whoop I am about to land in looks like. It would be nice if the third whoop in was evenly spaced, and allowed for a second and then a third jump out of the whoop section. Unfortunately, more often than not, when you land, you must drive from the valley of your landing to the tops of the whoops and drive through the remaining whoops.
On long sections of whoops, I just drive through the whoops, hitting only the tops. The problem here is that this is really tough to master. As I mentioned above, lose your nerve and chop the throttle, and your front wheel will drop into a valley and send you on your head. The only way to perfect whoops is to practice. Start with smaller sections and work up the the big stuff. I usually do a lot of passing in the whoops, as I figured out how to drive whoops years ago. I just wish I had the youth to keep the throttle pinned on the high speed sections.
Finally, once you master the whoops, the natural transition is to riding the scrub. In a race, few folks are going to be willing to give up their line so you can pass. As a result, you are foced into the scrub where there are rock, stumps and brush. There is a lot of risk passing in the scrub, but just use the same drive technique, and you will find yourself passing folks more frequently. Remember, everyone goes fast on the two track roads where it would be nice to pass.
Good luck.
Rick