The Rambam writes in Hilchos Teshuvah (3,3)
וכשם ששוקלין זכיות אדם ועונותיו בשעת מיתתו כך בכל שנה ושנה שוקלין עונות כל אחד ואחד מבאי העולם עם זכיותיו ביום טוב של ראש השנה מי שנמצא צדיק נחתם לחיים ומי שנמצא רשע נחתם למיתה והבינוני תולין אותו עד יום הכפורים אם עשה תשובה נחתם לחיים ואם לאו נחתם למיתה
The individual who falls into the category of בינוני  is not written in theספר החיים או ספר המות on Rosh Hashanah but rather hangs in limbo until Yom Kippur for him to decide his own fate. If he does teshuvah he will merit on Yom Kippur to be sealed in the Book of Life. If however he fails to do teshuvah, then he will be sealed in the .ספרי מתים  The sefer כוכבי אור asks on this Rambam why does this middle category have to do teshuvah in order to be sealed in the ספרי החיים. If tzaddikim who have a majority of merits are written to chayim, and the reshaiim who possess a majority of spiritual failures are written in the seferof missah, why doesn’t the Beinoni only need one more mitzvah to tip the scale in his favor to the side of merit? With this alone without the necessity of “teshuvah“, he can be written to Life?
The answer to this question given by many commentators is that the mere procrastination to do teshuvah during this time designated for teshuvahtransforms into a very severe aveirah which by adding mitzvos during this time cannot tip the scale. Every second one delays in doing teshuvah he is also negating the positive mitzvah of its fulfillment. Adding mitzvos cannot help veto the big aveirah of not returning to Hashem for past spiritual shortcomings.
The Meshech Chachmah brings this point out by comparing two similar pessukim , one in parshas ראה and the other in parshas נצבים. In ראה  the passuk says ראה אנכי נותן לפניכם היום ברכה וקללה. See that I am giving before you blessings and curses. In parshas נצבים  the passuk says ראה נתתי לפניכם היום את החיים ואת הטוב את המות ואת הרע. The first time Hashem says that if you choose not to fulfill the words of Hashem you will receive curses. The second time the same scenario is mentioned, you will receive death. What changed?  He answers that in the later parsha it mentions the mitzvah of teshuvah ושבת עד ה’ אלקיך. Once we are commanded to do teshuvah, the procrastination with its fulfillment delivers a harsher punishment of מות.
Another approach to answer this question is proposed by Rav Hutner ztl in Pachad Yitzchak Rosh Hashanah 18. The weighing of mitzvos against sins is not in the context of “quantity” and numbers. For if this was so, it would cause a difficulty in determining whether one has a majority of plusses or minuses since its calculations can fluctuate from one minute to the next. Rather, the judgment hinges upon one’s מדה שבנפש “quality”, which describes in which direction is one’s primary goal and gravitation. Is it towards spiritual growth or materialistic and earthly pursuits? We can even say that one might have a greater number of aveiros than mitzvos, yet his essential inner drive and ambition is to come close to Hashem. In this very scenario he is called the title ofרוב מצוות  which describes one whose purpose in life to acquire mitzvos. For instance is one is working to acquire the middah of patience. He has more failures than successes but he still doesn’t give up. He is an example of רוב מצוות  because his middas hanefesh is on a growth pattern of ruchniyus by bringing himself to such a commitment of constant challenges.
This is contrary to one whose only passion is for physical acquisitions and pleasures and that they alone constitute his ambition in life. For him, even if at the moment of judgment he has a greater count of mitzvos than aveiros, he is nevertheless not considered a candidate for the Chasimah Tova.
Now we can come to the בינוני . He has the nefesh middah of בינוני. He is such a person, who’s not committed to any identity, or life goal, not physical or spiritual. He is like a leaf in the wind, field dependent, “wishy washy” with no ID. He has ups and downs like a windshield wiper.  For him only teshuvah is the option in order to define himself to get a middas nefesh that is good, that warrants to be written with a chasima tovah. He has to make spirituality the drive and ambition in his daily life. Numbers of mitzvos and aveiros do not count in the judgment ofחיים ומות . Changed directions and lifestyles which come from changed attitudes and reframing of one’s mission on earth are what truly matters.
Most of us consider ourselves in the middle category. If so don’t let the yetzerharah fool us into thinking since I do so many chassadim and mitzvos I am not worried about being sealed in the Book of Life. Let us remember clearly that it is not about numbers. It’s also not about waiving a chicken over your head. With both of the above you haven’t changed your status. The only secure route that leads to a Chasima Tova is to get a completely new pair of lenses with which to look at Hashem, yourself, the world, and how to get off your Seesaw for real.

Gut Shabbos Gmar Tov
Rav Brazil