מאי קמשמע לן בשאין בה דין חלוקה – What is the משנה letting us know, that when there is no legal right to divide etc.
תוספות explains that the גמרא originally knew that the משנה is teaching us הרש"ה. Nevertheless the question is why teach הרש"ה by a חצר שאין בד"ח as opposed to a חצר שיש בד"ח. The גמרא answers that even if we knew that הרש"ה by a חצר שיש בד"ח, we may still think that by a חצר שאין בד"ח one cannot coerce the reluctant partner to build a wall.
כי רצו מאי הוי ניהדרו בהו – What of it, that they wanted; they can withdraw from this agreement.
According to the מ"ד גודא, there was never a question that they could renege on their commitment, for it is assumed that they made a קנין and pledged their assets for the wall. However, according to the מ"ד פלוגתא making a קנין for division is invalid since it is merely a קנין דברים. The מקשן did not entertain the possibility that they made a קנין ברוחות.
רב אשי אמר כגון שהלך כולי - Rav Ashi said that for instance, one went, etc.
רב אשי is teaching us that the קנין to divide can be either by specifically making a קנין to that effect, or by each of the partners making a חזקה in his share after they verbally agreed to divide the property (north and south).
The dividing wall between the היכל and the קדה"ק was called טרקסין for the following reasons; a) the word טרקסין means closing off (the לוחות which were given at) סיני; or b) the word טרקסין means inside and outside, indicating that there was a doubt as to the precise sanctification of the טרקסין space itself, whether it was קודש or קדה"ק.