When using a sinktip in a "slot" shaped pool, fishing deep into the drop off at the foot of rapids, or being limited to where you can stand to reach a good lie - it often helps to be able to set your line up so that it is parallel to the current flow, upstream and deeply mended... to avoid whiplashing the fly in the current. There are far more important and effective techniques but this one provides an effective method to set-up the basic swing (previous message) when you need it.
Essentially, the cast is made directly across or even upstream - but to avoid creating a sail in the line and whipping the fly downriver, a combination roll / mend is thrown into the line to set the line into a sharp upcurrent "hairpin" bend. The fly is able to penetrate the current because the line is laying with the direction of flow and the upriver bend takes the currents force. You can even half-mend the near side of the mend as it swings into position. By the time the line has reached "normal" position (above) the fly is much deeper having traveled from an upstream position into the "swing zone" without drag or whiplash. You can now apply the half-current speed tension and the fly swims in a much deeper plane than without.
For sinktips, you need to be careful not to snag from the extra depth charge.
One nuance is that if a fish takes while on the descent, you won't feel the direct take unless you are in the swing zone. Although rare that a fish strikes just then (as opposed to the swing or hang down), the ways I've found out that a fish has taken the fly in the past include line feeling 'hung up' and but a flick of the wrist to free the fly and suddenly the pool explodes.
Another is that you really aren't fishing the fly until you apply tension on the line... the initial part of the drift is for setting up the deep swing and not for hooking a fish per se. Of course I can recall several times when fish took on the setup and I didn't know it until the line tightened up with a twang! (but not as a general rule).
Next... finding and working steelhead lies. There's a joke there somewhere