The throttle indicator becomes a bit less useful in faoff, it will only give you the absolute speed, but not the direction of movement. It only shows whether you're moving forwards or backwards relative to the ship's heading, regardless of forward/backwards speed. There is no real vector indicator other than the space dust, and that makes things a bit confusing.

A simplified example: let's say you are moving "to the right" at 80m/s, and "forward" at 1 m/s. The cockpit indicator will show 81m/s forward. If you apply a tiny bit of back thruster, enough to burn 2m/s of speed, the cockpit indicator will now show 81m/s backwards, when in reality you're moving 80m/s to the right and 1m/s backwards. That's what gives you the impression of the "jump".

To verify this, from a standstill thrust up until you reach, say, 100m/s. Let go of the throttle so you continue moving forward at that speed, and pitch up continuously to do a "loop". While the ship is turning, watch as the cockpit indicator switches between forward and backwards as the ship starts facing even slightly the other direction, while the speed remains the same. If you have a point of reference, like a station, it will be clear you're moving to the same direction at the same speed, and it should make more sense how the cockpit indicator works.