its a system setting so yes it applies in all modes but obviously to varying degrees per mode. I guess it is actually most useful on a continuous but fairly gradual slope enabling you to stay in "trail" but to me it feels no different to changing to turbo in a low gear so always considered it something of a gimmick rather than anything usefull.I use trail with shuttle to help me up the longest steep climb on my regular ride. Yeah, I could switch to turbo for that bit of the trail, but my approach doesn't require a mode change. Not for everybody, but works for me. As congerball said, the key is to keep a high and steadyish cadence. You'll really feel the assistance die off if your cadence slows down or if your pedaling is on-off-on-off.
What I like about shuttle is that its extra assistance is essentially off when you are not pedaling at a high and steady cadence, but it kicks in automatically for the long steep steady climb. At least that is how it feels to me. I may be talking bollocks too!
I see it as analogous to an automatic transmission versus an old school manual. You want to manually shift every time, nothing wrong with that, whereas I've learned to like the automation. It's not essential, but something that comes in handy in certain circumstances. And not a gimmick.enabling you to stay in "trail" but to me it feels no different to changing to turbo in a low gear so always considered it something of a gimmick rather than anything usefull.
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