Re: sink tip casting help
Think about what a sinking line is. Most fly line is made of PVC doped with glass microspheres to help the plastic float. The higher the desired weight of the line, the greater the thickness of the line jacket (the plastic part). This is why 12-weight floating lines are so thick.
With a sinking line, instead of containing microspheres with air in them, the jacket contains a heavy metal dust, often tungsten or lead. On your better quality lines the amount of this dust in the jacket is varied (this is called density compensation) so the line doesn't sink in a big U shape because its densest part is in the middle of the head.
But since sinking lines can be made with a lot of variability in terms of how much dust and how heavy a dust the maker wants to use, they don't need to go up in physical size to achieve a higher line rating. Not much, anyway; a 12-weight sinking line is thus extremely thin--thinner than 5 weight running line usually.
Whether with sink tips or with full sinking lines, that extra mass (coupled with reduce line volume) is going to speed up the linespeed of the line. For someone not used to throwing sinking lines, that can cause "dumping," which is what happens when the loop of line unrolls or "turns over" before the loop has finished shooting out to your target. Dumping leads to inaccuracy and splash-down presentations that scare fish.
You need to work on dampening this tendency for the line to turn over so fast. One way to do that is to simply slow down. The sink tip cuts wind like a needle and a full sink line is even better. There's no need to overpower these lines; they will get to your target on their own.
The next tip is slightly technical: the specific cause of dumping that distance casters have hit upon is insufficient "overhang." Overhang is the amount of line between the tip top guide on your rod and the back of the rear taper on the line's head. The more overhang you can carry, the greater the "ball and chain" effect you'll feel as a caster. However, the more overhang you can carry, the longer you can delay turnover and thus dumping is easier to avoid.
Non-technical advice: try to gently shoot line in both directions while false casting. Just let a foot or two slip through your fingers as your loop is unrolling in the backcast, especially right before your final forward stroke. This will take some of the oomph out of your cast and help the sink tip act more like the line you're used to and less like a live wire.