use phent_addlocalforce with a nullvector as to where to apply the force.

Depending on your friction, your force may not be enough to move you and when you move it may keep you at const. Velocity. With no friction, you would continue to gain velocity as long as you continued appling a force.

To stop, simply increase the Drag dramatically...I've found this to be a good way to stop a physics ent.

Finally, to jump, use the same phent_addforcelocal, but with a vector like (10,0,100), which tells it to apply a force forwards and up, a jump!