The difference is that it propelled a stone ball instead of a large arrow.
The  ball  was  driven  along  a  square  wooden  trough,  one-third  of  the 
diameter  of  the  ball  being  enclosed  by  the  sides  of  the  trough  so  as  to  keep  the missile in true direction after the bow-string was released.

The bow-string was in the form of a broad band, with an enlargement at its 
centre against which the ball rested.

The description given of the mechanism and management of the engine for 
throwing arrows can be applied to the construction and manipulation of this form 
of ballista, which was also made of large and small dimensions.

Small engines with arms about 2 ft. in length and skeins of cord about 4 in. 
in diameter, such as those I have built for experiment, will send a stone ball, 1 lbs. 
in weight, from 300 to 350 yards.

There is little doubt that the large stone-throwing ballista of the Greeks and 
Romans was able to project a circular stone, of 6 to 8 lbs. weight, to a distance of 
from 450 to 500 yards.¹

A.  Surface view, with the stone in position.

B.  Side view, with stone in position.

C.  Front  view  of  the  stone  as  it  rests  in  the  trough  against  the  enlarged 
centre of the bow-string.

D.  Enlarged view of the solid end of the sliding trough. This sketch shows 
the ball in position against the bow-string ; the catch bolding the loop of the bow-
string, and the pivoted trigger which, when pulled, releases the catch. One of the 
pair of ratchets which engage the cogs on the sides of the stock, as the trough is 
drawn back by the windlass to make ready the engine, is also shown.  The trough 
has a keel to it, and slides to or fro along the stock in the same manner as in the 
arrow-throwing ballista. (Fig.13.)

Compare with figs. 13, 14,, for further explanation of details.

¹  The balls used by the ancients in their catapults and ballistas were often formed of heavy pebbles enclosed in baked clay, the reason being that balls made in this way shattered on falling and hence could not be  shot  back  by  the  engines  of  the  enemy.  The  ballistas  for  throwing  arrows,  and  those  employed  for  casting stones, were fitted with axles and wheels when constructed for use in field warfare.

From ‘Il Codice Atlantico,’ Leonardo da Vinci, 1445 – 1520
Criticism. – A stonebow of vast size.  A and B represent two kinds of lock.  In A, the catch of the lock over which the loop of the bow-string was hitched, was released by striking down the knob to be seen below the mallet. In B, the catch was set free by means of a lever.  C shows the manner of pulling back the bow-string. 

By turning the spoked wheels, the screw-worm revolved the screwed bar on which the lock A traveled.  The lock, as may be seen, worked to or fro in a slot along the stock of the engine. In the illustration the bow is  fully bent and the man indicated is about to discharge the engine. After this was done, the lock was wound back along the screw-bar and the bow-string was hitched over the catch of the lock preparatory to bending the bow again. 
Besides being a famous painter, Leonardo was distinguished as an inventor and exact writer on mechanics and hydraulics. 

‘No  artist before his time ever had  such  comprehensive talents,  such profound skill or so discerning  a judgment  to explore  the depths of every  art or science to which he applied himself.’ – JOHN GOULD, Dictionary of painters, 1839.

From the above eulogy we may conclude that the drawings of ancient siege engines by Leonardo da Vinci are fairly correct.