Perhaps you're referring to "slide" as Tachyon called it, but technically in FS2 it's called "glide."
Slide/glide will "pause" the engines so that you continue on the same course but can rotate the ship in place.
Lats or lateral thrusters, cause you to go left or right while still maintaining the forward or reverse speed.
FringeSpace (our mod) uses slide and lats as major movement features, but I've not gotten a chance
to renovate things so we use Sushi's new slide functions. BTRL might have used some lats or glide, I forget honestly.
Here's something to start with as an example:
$Damp: 0.0 ;; affects how quickly you will accel/decel to your target velocity. Higher damp means slow acceleration and deceleration - lower this number, the faster the ship responds. Example, specifying a value of 0.0 means there is no damping, in other words, "like Wing Commander" The more damping, the harder to control, but the more smoothly it moves.
$Rotdamp: 0.1 ;; same as Damp except for rotation (all axis I believe)
$Max Velocity: 30.0, 0.0, 74.13125 ;; in x/y/z format x = lats y = verts (N/A in tach) z = forward. use special tokens for backward movement - all values are added together to make the total max speed of the engine at 100% thrust IE. 25,25,100 means throttle at 100% is 150 MPS, NOTE - anything over 800 total seems to break collision detection
$Rotation time: 4.05, 4.17, 3.76 ;; x, y, and z -axis - number of seconds it takes to complete rotate a ship (360 degrees) around the given axis
$Rear Velocity: 74.13125 ;; maximum reverse velocity in meters per second - is not added to maximum velocity count
$Forward accel: 8.13 ;; number of seconds needed from full stop to maximum velocity
$Forward decel: 7.86 ;; number of seconds needed from maximum velocity to full stop
$Slide accel: 1.02 ;; Lats & Verts - number of seconds needed from full stop to max lat/vert speed
$Slide decel: 3.22 ;; Lats & Verts - number of seconds needed from max lats/verts speed to full stop
$Glide: YES ;; Allows (Tach: 'sliding') (FS2 'gliding') yes or no
It's not perfected by any means, but might help you in some ways as an example.