Quoted from Mr_Outlane:
Backhanding the secret passage has been problematic for me since day one. There's about a 10% chance I'll pull it off. This has more to do with the resistance the ball encounters in that steep turn on it's way to the upper roll over switch (circled below). I don't think increasing flipper strength will solve the problem. But it may help. Backhanding the crypt has always been tough for me.
I was able to backhand the secret passage with ease on day 1, and stayed that way for the first 10-20 days of this NIB pin. It became more and more difficult with each passing day, and now I can also make it maybe 10% of the time.
Perhaps it has more to do with flipper strength?