Hogan "looked" flatter because he had SUPER FLEXIBLE wrists that gave him a whole lot of wrist cock with very very little arm swing.
Also, i'd also recommend to learn how to trace a straight plane line on whatever plane you naturally swing on. If you are trying to change what plane angle you swing on just "cuz" you will ruin your swing long term.
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I'm not a TGM or PGA certified Pro, but I did stay at a Holiday Inn Express last night
I think most shift to the turned shoulder plane or the squared shoulder plane, either with a single from elbow upward to the 'shoulders' or even double returning back to elbow. How the hands are set at address will help your chosen swing plane, too.
I often hear that Watson "single planed" on one of the shoulder planes but watch the vids (although time changes people swings), he started on elbow and single shifted (quickly) to the shoulder, turned , I think.
Don't get trapped into thinking a shift is no-no. If it feels right, do it.
Regarding Toms, the sweetspot stays pretty close to the turned shoulder plane throughout (just slightly higher at End), but the shaft starts out much flatter at address. Wouldn't this be a single shift, or does one reference the sweetspot when talking about plane shifts?
Don't get trapped into thinking a shift is no-no. If it feels right, do it.
I share the same sentiments as Mike here. At times, I do teach a shift to the elbow plane if it helps the student drive the right shoulder down the plane instead of around it.
It's like what Peter Drucker used to say, "Manage by exception, not by rule."