When people talk about stacking power siphon, they refer to the buff that it provides. This is easy - just use five attacks in less than ten seconds and it's five-stacked. I could be mistaken but I don't believe you actually get any benefit from stacking power siphon itself - better to fire it off as soon as it wears off to extend the chain.
When it is activated, each attack in that 20second duration adds or "stacks" onto its buff. So if you activate it and get 5 attacks off in a short enough span of time, each attack applies a buff, and it will have stacked 5 times. Reusing Power Siphon itself doesn't cause any extra stacking AFAIK.
Thanks a lot for the quick replies!
I got it all wrong, I didn't realize the stacks originated from my atacks during Power Siphon. Sorry for my noobness 
Hello everyone!
I just started playing and chose a KM/Elec as my first Hero. After finding out that Concentrated Strike Criticals instantly recharge Power Siphon, I chose to play as a Scrapper.
But after some research and playing with Mids, I can't see how am I gonna be able to stack Power Siphon 5 times. Please, let me know if my thoughts below are incorrect:
After getting Hasten, Lightning Reflexes and slotting 3 Recharge SOs on Concentrated Strike, I came up with a 7 seconds recharge. And AFAIK, Scrappers have a 10% critical chance on any mob besides Minions.
Even if I get several Critical Strikes in a row, I would be able to keep only 3 stacks of Power Siphon up since its duration is 20s (3x7=21). Did I miss something here? How can I stack it 5 times?
Thanks in advance