Hamifaneh libo l'vatalah. Rashi writes l'hibatel midivrei Torah. What is he saying more or different than what he said earlier, on hane'or balalah - "umichashev b'libo divrei havai"?
Also why does the Tanna use the term hamifaneh libo as opposed to the well known 'mivatel midivrei Torah'?
It is also not clear what the chiyuv bnafsho here is. - is one who batels chavav misah? The Mishnah immediately before this just says moshav leitzim, & a meal without Torah is like eating from idol sacrifices. - none of these are chayav misah!
R' Yonah seems to learn that its not an actual chiyuv misah. Rather, one who wastes his time & life is relinquishing his 'right' to live - he is 'indebting' himself to death.
On a similar but slightly lighter note, R' Yakov Kamenetzky writes that nafsho means is mind. One who makes no use of his mind when he has the opportunity mischayeiv himself to lose its ability. - (quite a fearful thought when one thinks what may be a cause of alzheimers.)
There is an interesting Chassid Ya'avetz that says we should be very aware of the multitudes of spirtual forces that fill all the space from here to the heavens - both pure & impure forces.
The impure forces are attracted to vacuums of potential holiness & this is the warning of our Mishnah.
Along these lines, the Maharal explains that specifically nighttime & 'derech' are the times that one is unsafe from these forces. One who leaves the norm of society that are inactive at night is not protected by the general sh'mirah that Hashem provides to the masses. Adam HaRishon designated places for yishuv bnei adam, & the places he did not designate remains the jurisdiction of 'chayos', - both physical & spiritual. One who excludes himself from normal yishuv haolam is in great danger indeed & requires the added protection of Torah.
The Chassid Yavitz explains, that is why our mishnah speaks of One person who is idle - because here we are not discussing the sin & punishmant of bitul Torah - which certainly is worse when two are involved. Rather the Mishnah speaks of the inherent natural danger that these forces present to a lone idle minded person. Two people would normally at least converse mundane matters regarding business, family, health etc. This although it may be bitul Torah, but with regard to the danger our mishnah discusses, includes them in 'yishuv ha'olam'.
Based on this approach, I believe Rashi is answering another question, while also offering a slight leniency to this very sobering Mishnah.
If we are not discussing the sin & punishment of bitul Torah, rather a natural danger & consequence of the 'idler', why doesn't the rule of ''torah migna bein bitna bein shelo b'idna'' - "Torah protects whether during or not during actual study"?
This is why Rashi explains that here we are not talking about one who just happens to be daydreaming on his walk or during the night. Perhaps he can still merit the Torah's protection. Rather we are discussing someone was MIFANEH libo L'HIBATEL" - TURNED (OR EMPTIED) his heart to be empty from Torah".
This person does not merit the Torah's protection. It is a complete disassociation from torah for the time being & that doesn't allow the Torah to be maigen.
We must a) use opportune times for Torah & b) we must be extremely wary of nightime & roadtime idleness. c) One who sets himself up for a purposeless nighttime "browse'' is cetainly fulfilling all three approaches to the dangers of our mishnah. Hashem Yishmor.
