When you draw with a pencil on paper, the paper you chose can either be soft and very easily dented, or it can be hard with a smooth resilient surface. The pencil you use can be a very soft 6B pencil or it can be a very hard 6H fine pencil.
If you press down on the pencil hard, or use the pencil softly it will make a difference to the quality of mark you make and the indentations made on the paper. Experienced, highly skilled artists can create bold, competent and assured marks and have less fear of mistakes.
Once you make an indentation on the paper it is very hard to work over it and hide the marks with deeper the indentation. some marks may never be possible to completely obscure. If you rub out a strongly mistakenly made line, it will never completely disappear and will always spoil the final artwork.
So it is with horses. Your horse is the paper, your training tools are the 6B or 6H pencil and your choice of how softly or hard to press is your training approach.
Once mistakes are made, the deeper the marks, the harder they will be to work over them and create beautiful Art.