Ask why one e-bike feels natural and another feels like a light switch, and the answer is usually the pedal-assist sensor. It is one of the least-advertised specs and one of the most important to how a bike rides.
Cadence sensor: on or off
A cadence sensor simply detects whether the pedals are turning. Start pedalling and the motor kicks in at the set level; stop and it cuts out. It is cheaper and common on budget bikes. The downside is a slightly delayed, all-or-nothing feel: the power arrives a beat after you push and does not respond to how hard you are working.
Torque sensor: it reads your effort
A torque sensor measures how hard you are actually pushing on the pedals and matches the motor to it, instantly and proportionally. Push harder, get more help; ease off, and it eases off. The result feels like your own legs, just stronger. It is more responsive, more intuitive, and easier on the battery because the motor only gives what you need.
Which should you look for
If you can, choose a torque sensor. It is the single biggest factor in how premium an e-bike feels to ride, often more noticeable than the motor brand. Most quality mid-drives use one; many budget hub-drive bikes use a cadence sensor. If a bike feels jerky on a test ride, the sensor is usually why.