Plenty of good stuff on here, so I just have about 0.5 cents to add. On speed, if you are worried, do it within reason. Bombing a jib feature can lead to unintended consequences and chances are most people instinctively slow down and the first try will be like (bah! wish I had more speed). The other thing with speed is the line you take to go into a feature. Think about it before you hit a feature, bearing in mind you have some kinks to go thru. And how much speed depends on what kind of kinks. But the foward vs downward momentum things is true.
Related to that above is like some have said you need to change your body positioning as you hit the kinks. This can be your shoulders, to how you angle your torso to be 90 degrees above a feature, to ankles which are IMO the most important cos they probably are the part of you that react the fastest and the most direct (but not only) factor influencing the edge angle you put onto a jib.
As for the unweighting issue, you see MANY riders do it. Especially on down flat down features or up flat up features cos there is an obvious angle when approaching a flat section where your board can catch. For straight up flat-down features where the kinks are just downwards, the problem is not so much catching but slipping out backwards cos you dont lean forward enough to match the gradient. Unweighting also becomes important when you do transfers on a kinked rail, like backboard-front-board-whatever out etc.
Last thing is the unweighting period is usually really short but critically important. Cos all your body adjustments like shoulder alignment, (counter) rotation, ankle flex are ideally done when unweighted so you come back down on the feature after a split second ready to continue to the end of the feature and stomp it nicely (which is the punctuation mark at the end of the sentence when it comes to jibbing).