The trainer
Four modes that build on each other: browse the line, learn why each move is played, practise it with nothing in front of you, then prove it against the clock.
Browse puts the whole line on the board so you can look at it. No quiz, no timer. This is where you notice the shape of the position: which pawn is committed, where the pieces want to go, which side the play is on.
Learn then walks the line move by move and explains the moves that carry the idea, in plain language rather than a wall of notation. Knowing why a move is played is the part that survives a real game.
Practice mode takes the training wheels off. The board asks for the next move and you have to produce it, and a wrong move is shown immediately while the position is still in front of you.
It feels slower than reading, and it is meant to. You are building recall, which takes longer to form and much longer to lose.
Every course can be trained as White or as Black, and you can flip at any point. The side you skip is usually the side that beats you.
Time trial is Practice against a clock. A line you can produce slowly is a line you do not have yet, and the clock is what exposes the difference. It needs an account because the result is kept against your progress.
Not to browse, learn, or practise - and the Fried Liver Attack course is open to guests entirely. An account is needed for Time trial, and to save progress to your profile rather than just your browser.
No. Learn mode attaches a reason to the moves that matter, and Practice then asks you to reproduce the line unaided. The point is to understand the idea well enough to play it when the position arrives in a slightly different order.
Short courses like the Fried Liver can be finished in a single sitting. Larger ones such as the Najdorf are split into named lines, so you can work through them in pieces and still see how much of the course you have done.