Its just a way to stop the dog and get his attention. He'll be waiting for the next command, which is whatever you desire.
When hunting upland that could be to move him into another direction with hand signals.
Or when retrieving, to stop him and get his attention to send him off in a direction.
Everything is done in steps and its good you have him on the whistle before starting on hand signals.
Get the obedience down before you move on.
Don't jump the gun on hand signals during retriever training till hes steady on retrieves and delivers to hand.
Easiest way IMO to get the hand signals down is the imaginary baseball infield. Your at home plate, dog sits and stays pitchers mound. Use 1st and 3rd base for left and right directions, 2nd as "back". Toss dummy at any base and use exaggerated body English in the direction as you send him off.
You can see why he needs to be steady before you start this.
Get it mastered then move on to blind retrieves. At heal give him the line then send him off. Stop him with the whistle then give him the direction.
A good book or DVD will have all this info for you. Techniques might be different.