Just to be clear, the +/- button in the calculator is the negation key, so following the keypresses in the tutorial means that you entered the value -100%.

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This is not easy to explain with just text but let's try....

- to my understanding:

An Acceleration of 0% represents a linear transition to the next step, the exit vector, has the same angle as the direction to the next stepvalue.

An Acceleration of -100% represents a ease-out towards the next stepvalue, the exit vector is horizontal.

A Deceleration of 0% represents a linear transition from previous stepvalue, the entry vector has same angle as direction from the previous stepvalue.

A Deceleration of -100% represents an ease-in from previous stepvalue, the entry vector, is is horizontal.

When entry and exit vector both are horizontal the transition is smooth, as a sinewave

Acc/Dec with even lower value than -100% represents direction vectors further away than horizontal, in other words in the wrong direction of the target.

Acc/Dec with positive values represents the equivalent vectors on the other side of the vector of the linear transition, representing a more aggressive entry/exit

for testing try the following with some fixtures selected:

At 75

Next Step

At 25

Step 1 + 2

Accel -100

Decel -100

Accel -400

Decel 200

Accel 100

Decel -300

When the sum of Accel and Decel = -200, the entry and exit vectors are parallel

When entry and exit vector are parallel, the transition is stepless/smooth

The math behind is bezier curves, the valueranges are normalized for easier interaction.