A smart trainer can hold you to a workout in two quite different ways, and most riders use one of them without ever having chosen it.
ERG mode, where the trainer decides
In ERG mode you give the trainer a power target and it does whatever is needed to keep you there. Pedal slower and it adds resistance, pedal faster and it takes resistance away, so your power stays at the number regardless of what your legs do.
What you get is precision. If a workout asks for 4 × 8 minutes at a given effort, ERG delivers exactly that in every interval, including the last one, where you would otherwise have quietly let twelve watts slip away. For threshold and sweet spot work, where the point is time spent at a specific intensity, that is genuinely valuable.
What it costs you is pacing practice. A rider who only ever trains in ERG has never had to hold a number themselves, and pacing transfers directly to events.
Slope mode, where you decide
In slope mode the trainer simulates a gradient and then leaves you alone. Resistance depends on your speed and the simulated hill, exactly as on the road, so if you want more power you push harder or change gear.
It suits anything where the effort is meant to vary, along with sprints, practising a pace, and any ride that should feel like riding rather than being held in place. It is also the honest way to ride a test, because no ceiling is imposed by a target.
What it costs you is discipline. Nobody holds a steady number as precisely as a machine, and on a long interval your attention wanders.
The spiral of death
This is the failure that makes riders hate ERG mode, and it is avoidable.
You are in ERG on a hard interval. You tire, so your cadence drops, and the trainer adds resistance to hold power at the target. The extra resistance makes the pedals harder to turn, so your cadence drops further and the trainer adds more resistance. Within a few seconds you are grinding at forty revolutions a minute against a wall, and then you stop.
It is a feedback loop, and once it starts you rarely pedal out of it.
Avoiding it. Stay on top of your cadence rather than letting it sag, and if you feel it falling, lift it deliberately before the trainer reacts. Choose a gear with room in it, roughly the middle of the block, so a shift is available. And if the interval is genuinely too hard, lower the target rather than fighting the machine.
Escaping it. Most trainers recover if you stop pedalling briefly, which resets the resistance, or if you shift gear, which many treat as a cue to re-evaluate. Either beats abandoning the session.
Which to use, session by session
| Session | Mode | Why |
|---|---|---|
| Threshold and sweet spot intervals | ERG | Time at a precise intensity is the whole point |
| Steady endurance | Either | Slope feels more like riding |
| VO2 max intervals | Either, with care | ERG holds the target, but the spiral bites hardest here |
| Sprints and short efforts | Slope | ERG cannot respond fast enough to be useful |
| FTP or ramp test | Slope | A test must let you exceed the target, and fail honestly |
| Practising pacing | Slope | The skill is holding the number yourself |
Why a test has to be ridden in slope
A test ridden in ERG at a fixed target cannot measure anything above that target. You will finish having produced exactly what you were told to produce, which tells you nothing you did not already know.
A test has to let you exceed the number if you are stronger than expected, and to let you stop when you are genuinely finished. WattPlan's FTP tests are built to be ridden that way, so the result reflects what you could actually do rather than what was asked of you.