Lyman
As you suggest jet engines are very much Sir Isaac in action.
With the Harrier in the hover the thrust and hence Sir Isaac are supporting 100% of the weight.
As the nozzles are rotated aft the vertical component of thrust reduces as the sine of the angle.
As IAS increases the wing provides an ever increasing lift (as with any other wing) helped by the fact that it has an angle of attack of some 8 deg with the aircraft in the hover attitude. The hover pitch attitude is determined by the undercarriage and the need to land mainwheels first to avoid overstressing the nose leg.
The above diag shows how trig works in the pilot's favour when stepping off from the hover. The first 15deg of nozzle rotation produces a .26g accel while only costing some .03g downwards. Something for (nearly) nothing!
The pilot rotates the nozzles at a rate that gives him his chosen accel path - too fast and a sink will occur - too slow and height will be gained. With a level transition the nozzles will be aft after about 13sec.
All extremely easy to do.