Here is a short description about how tolt is ridden.
Tölt II.  
How tölt is ridden. 
By Arnthrudur Heimisd
It is difficult to discuss how to ride tolt, as individual horses are very different.  You learn much by using every opportunity you have to try riding different icelandic horses, and trying how you have to change your riding in many minute ways riding them. The same horses can have, and usually have, different footfall in the tolt depending on many things, like how they are ridden, in what shape they are, on what kind of ground they are tolting, whether they are riding up or downhill, how fast 
they are going, and more. 

As the footfall is similar in walk and tölt, you often let the horse tolt from walk.  As the horse carries its neck higher in tolt than in walk, you shorten the reins a bit before and while the 
transition is done.  Usually it is easier for the rider to tolt the horse if the rider sits a bit backwards in the saddle, that is, sits maybe an inch or two behind the point where he usually sits.  The rider has to take care not to tilt backwards, the legs and back should be straight as usually, and relaxed. So, in the transition from walk to tolt, the rider: 

   1.Moves a bit backwards in the saddle. 

   2.Shortens the reins. 

   3.Encourages the horse to go faster, with a verbal clue and with the 
      lower leg. 

   4.When the horse has tolted a few steps usually you give it again a 
       bit of rein (an inch or so), so it can move freely in the neck, but keep 
       necessary reincontact.   
      The hands should be like rubberbands, have reincontact without  

So that the horse can tolt well, it needs freedom in the withers (that is why you move the weight backwards and encourage speed so that the horse powers from its behind, and that is also partly why goey horses are popular in Iceland, this is more natural for them).  It also needs to carry its neck rather high (I need an english word for this, but this picture that shows this here below) 
But the faceline may not be too horizontal, or 
too vertical.  The horse is collected, not like a dressage horse, but 
collected anyway, using their behind as a motor to push 
the light front end forward, free the withers and allow the horse to 
balance it self, not lean on the reins.  If the horse does 
not know how to collect, teach it collection at the walk, and later 
(weeks later) try keeping that collection at the tolt. 

How the tolt is ridden in Iceland.  The posture of the horse is good, 
the head and neck is carried excellently, but the 
feet of the rider are a bit much forward (that is very common here and 
doesn't spoil the horse's performance, just 
looks bad) and the rider has contact with the reins.  You want the neck 
of the horse to elevate, not the nose. 

Find the ideal speed for your horse to tolt clean (or almost clean). 
All horses have a speed where it is easiest for them to 
tolt clean.  For pacy horses this is usually medium-speed, for trotty 
horses this is usually slow or fast tolt.  As they get 
more training, you can tolt them slower and faster than this particular 
speed without loosing clean hoof-beat.  But this is 
also the reason that it is often problematic to tolt-train horses with 
bad tolt-balance in a group, because each of them 
might need to tolt in different speed to be at their best. 

Find the ideal ground for your horse to tolt on.  Usually it is best 
where the ground it not very soft, it is more difficult for 
the horse to tolt as the ground gets softer.  Keep though in mind that 
tolting for long distances on asphalt is straining for the 
legs of the horse. 

The horse needs to be soft in the mouth to tolt well, do everything you 
can to keep your horse soft and responsive in the 
bit.  Avoid a dropped back and eve-neck, because that leads to a tense 
body and stiff or no tolt.  The softer you are, the 
softer the horse is, and the softer the tolt is. 

The saddle needs to fit well for the tolter, and sit right, or he 
stiffens up to brace himself against pain. 

Many horses tolt better going slightly downhill (again, extremes are 
bad).  It is difficult for all horses to tolt clean uphill, 
and riding a horse uphill in tolt either teaches the horse nothing about 
tolting or makes the tolt worse.  Tolt (or walk) on 
horizontal or downhill, trot  (or walk or canter) uphill. 

Improving a horse that lacks balance in the tolt takes time in many 
cases.  Be patient, good things happen slowly, and do 
not get frustrated even though the training takes weeks or even months. 
Teaching a piggy-pacer to tolt can take riding him 
4-5 times a week for 3-6 months.  If it happens fast, be overjoyed, but 
brace yourself for a long training period.  Give 
your horse at least 3-4 rides per week for 2 months if you really want 
to change it's tolting. 

Training pacy horses and trotty horses will come in later articles. 

                                    Happy tolt-trails