What is the difference between שליחות יד and גזלן?
Tosafos ד"ה שנטלה writes that if the שומר takes it to steal it he is חייב because the גזילה itself is a חסרון. Why did Tosafos need to say that? Just say that he is a גזלן? We see from Tosafos that a שומר can never become a גזלן because he is an extension of the מפקיד and therefore the חיוב is always from שליחות יד.
Rashi asks what is the difference between שליחות יד and שואל שלא מדעת?
The Baal Hamaor and the Raavad hold that there is no difference, they are interconnected, if שליחות יד doesn't need חסרון then it is the same as a שואל שלא מדעת.
The Tur (סי' רצ"ב ועיי"ש גר"א) answers Rashi's question differently. The Tur says that שליחות יד requires that the use you are going to make of the object will cause a חסרון (based on the Gemara). When a person is שולח יד he is חייב as soon as he is מגביה however a שואל שלא מדעת is not חייב from when he picks it up but rather from when he actually uses it.
Take the following example. Reuven is watching Shimon's barrel. Reuven decides that he is going to use the barrel as a ladder. Using it as a ladder causes no חסרון and therefore he is not considered to be a שולח יד but rather a שואל שלא מדעת. He picks up the barrel and moves it to where he needs the ladder. Then Reuven gets a phone call and doesn't actually use it. In the meantime, an אונס happens and the barrel breaks. According to the Tur, since Reuven is only a שואל שלא מדעת, Reuven is פטור because the אונס occurred before he actually used the object.
The Ramban answers Rashi's question that the difference is that a שואל is defined as someone who gets all the הנאה, however, in a case where he is not getting all the הנאה but the original owner is still getting some of the benefit it cannot be called שאילה and therefore would fall under the category of שליחות יד. Here in our gemara, where he places he stick etc. on the animals back, the original owner is still benefiting from the שמירה and therefore it can't be called שאילה rather it is שליחות יד.