Mechanical failure vs all out failure
Mechanical failure
Completely exhausting the targeted area while maintaining perfect form, bio mechanics and control without the recruitment of potential assisting muscles.
All out failure
Exhausting the targeted area up to a certain point until assisting muscles help aid in the completion of the exercise causing less overall specified muscle fiber recruitment which will hinder that muscles potential to grow, this style of training also increases the risk of injury. When you don’t focus on the area your trying to target but yet just moving the weight its a lot easier because its “by all means” which in turn might be more in terms of overall weight used but it will be less in terms of actual desired muscle tissue growth for the area your trying to target.