As a general rule Industrial Linear Actuators will not back drive, these will have a static load capacity listed in the catalog. Precision Linear Actuators generally will back drive by themselves and rely on the mounted motor to hold a load.
Backdrive is when the load of a screw, actuator, or system is able to operate the unit in reverse when unpowered. In most cases with Thomson products, this means that the force of gravity drives the nut down a screw, retracts an actuator, or moves a slide when power is not being applied.
Thomson prevents backdriving through various means on Industrial Linear Actuators. The ACME-style actuators (Electrak 1 & 2) use self-locking modified ACME nuts.  Ball screw actuator models incorporate anti-rotation holding brakes.
One exception to note is the Thomson Max Jac, which does not include a holding brake. The ball screw version can backdrive under moderate loads. Certain other Thomson actuators may be produced without brakes based on customer requirements.