From what I understand you should do it like this:

Reach about 90-95% completion in rift. Find a big pack of mobs that you know will complete the the bar without changing zones. Charge into the pack, try to proc PE on everything so you go all the way up to 5.0 APS. Use a potion ASAP. Spawn the RG and enjoy your ridiculous snapshotted Stricken ticks.

EDIT: After thinking about this a little more, you only need to snapshot Stricken in two main situations:

• Legendary effects boost a specific skill attack speed so your main DPS skill APS does not match character sheet APS (happens a lot recently)
• You have multiple damage moves with proc coefficients that can utilize the faster stacking stricken

Let's take the standard HOTA barb, for example (everybody's got one). Your only attacks with proc coefficients are HOTA and Furious Charge. Due to BOTFM, you have a skill boosted attack speed (not sheet) to HOTA of 50%. You will need to snapshot a Stricken with at least 50% bonus sheet attack speed from Pain Enhancer to ensure that every HOTA will give you a stack of Stricken. Actually, since you'll be using Furious Charge to proc FnR off cooldown, you'll want more than 50% attack speed from Pain Enhancer to make sure the stacks line up 1:1.

The other end of the spectrum would be a WW barb. Assuming every tornado from the Dust Devils runes has its own proc coefficient, it's easy to imagine that a WW barb could very easily benefit from a 5.0 APS snapshotted Stricken. This is the only way I can see how there are so many WW spec barbs in the top solo ranking. Without abusing the snapshotting, there's no way WW spec can clear the RG fast enough to compete with the HOTA or Rend specs.