Back in the days of friction, you had nothing to push and lift the chain onto a larger cog than the strength in your thumb, the stiffness of the derailleur cage, and the cross-section of the teeth on the chainring. Which is probably why I saw so many bent chainrings in those days. And why a PX-10 with Stronglight 93 cranks shifted better than a UO-8.
With ratcheted shifting the pull on the cable comes from the shifter, which isn't necessarily as strong as your thumb. That means you can't force the shift if it isn't happening, but it also means less chance of damaging the chainring. The pins, ramps, gates, tooth cross section, and shapes of the derailleur and outer plates on the chain work together to push and lift the chain onto the big ring. The derailleur shoves the chain over and the landscaping on the chainring lifts it over the top. I've spent hours watching this in slow motion, looking for exactly where the chainring got bent, the ramp got worn off, the pin got nicked or loosened or knocked out, the tooth got damaged, or the chain got kinked or a stiff link. It's all about doing what you have to to get the bike off the stand so it won't come back.
The cool thing about working with a good bunch of mechanics is, you can say, "Can I borrow your eyes for a minute? Watch this and tell me what I'm not seeing." Some customers will tell you you're **** if you can't figure it out right away, but if you're thorough and you help, share, and ask questions, the guys (and girls) you work with have your back.