Tacx Antares Professional Training Rollers. These heavy-duty tapered rollers ensure that it is almost impossible to ride off the machine. High concentration on steering is required when riding without a brace; this reinforces your sense of balance.
However, they would be a far better product if they had a revolving washer as do true track nuts. That would make fitting and then aligning the back wheel straightforward.
As they are, they have to be fitted with a washer which doesn't come supplied, or you'll have to fit them one at a time, replacing one track nut at a time and keeping the axle positioned with a very tight track nut as you put the opposite side tacx nut on, and then when the first tacx nut is really tight with the axle still in position, take off the opposite track nut and out the other tacx nut on.
Even then you'll have to tighten the second nut and keep the wheel straight.
If you don't do it this way, the back wheel either won't align straight or you'll end up unwinding the axle from the bearings and have one side longer than the other with grease leaking out.
Serrated faces on the nuts might help as an alternative to revolving washers, but washers work better and don't chew up the dropouts as much.