It states (Tehillim 13:6) לִבִּי יָגֵל בָטַחְתִּי ָבְּחַס ְדְּך וַאֲנִי ָבִּישׁוּעָתֶך, "I have bitachon in Your kindness. My heart rejoices in Your salvation." With these words, Dovid HaMelech expresses that he is happy with the salvation, even before the salvation comes. He has bitachon and is certain that Hashem will help him, so he can rejoice immediately.
We can compare it to when a poor person receives a check for $100,000 from a wealthy person. He is happy right now because he knows that all his debts will be covered. Upon seeing his ecstasy, someone asks him, "Why are you happy now? You don't have the money yet. All you have is a check!" The poor man explains that he has the check in his hands, and he is certain that he will soon have this money. He doesn’t suppose the check will bounce.
Similarly, and endlessly more so, a person can be sure of Hashem's salvation. Even before the salvation comes, he trusts in Hashem, and he rejoices in the upcoming yeshuah.
The Rebbe of Toldos Aharon zt'l was once on a vacation (either in Switzerland or in Teveria) and was talking about the people who buy insurance (which people call בטוח in Eretz Yisrael). He discussed life insurance, health insurance, fire insurance, and so on. He said, "I understand these people who buy insurance. They are afraid that something terrible might occur, and they want to be insured..." He spoke about this for several minutes, and then the Rebbe said, "I also have insurance (ביטוח)." The people listening were surprised to hear that, and they wanted to hear which company he uses and what type of insurance he has. He said, "My insurance is רחמיך על כי בטוחים אנו הרבים, 'We trust in Your immense compassion.' I sleep well, I have no worries, because I know that I have insurance. My insurance is Hashem's compassion." (Written in Zechor l'Avraham, quoting from one of the students).
In Yedid Nefesh we say, ותתרפא תתחזק אז, "Then we will be strong and healed." We can explain that תתחזק means that if we are strong with bitachon in Hashem, תתרפא, we will be healed, and we will receive salvations (Birkas Avraham, letter 65, quoting his father, the Divrei Shmuel).
My father-in-law, the Zutchke Rebbe zt'l, teaches (Yeshayah 1:1) אָמוֹץ בֶן ַׁע ְ יָהוּיְש חֲזוֹן, which can be translated, ַׁע ְ יָהוּיְש חֲזוֹן, if you want to see your ישועה, salvation, אָמוֹץ בֶן, strengthen yourself with bitachon.
The Baal Shem Tov zy'a teaches that when Hashem sends yesurim to a person, Hashem first causes the person to become sad. (Degel Machaneh Efraim, Ekev, מצרים מדוי כל ה"ד). Similarly, the Toldos Yaakov Yosef (Mishpatim ואלה ה"ד) writes, "I heard from my teacher, the Baal Shem Tov, that when Heaven wants to punish someone who deserves punishment, first, their level of bitachon is taken away. Therefore, a person should daven that he be strong with bitachon."
The Klausenberger Rebbe zt'l (Shefa Chaim, Chumash Rashi, Bishalach, ב"תשמ) says, "When a person is in a great tzarah, and he understands that according to the rules of nature, no person, and no counsel can help him, only Hashem, alone... that is when he will certainly receive his salvation. [This is because he knows that only Hashem can help him, so he places his trust entirely in Hashem.] All he must do is wait silently, with emunah. His emunah will help him more than tefillos. But as long as a person thinks that the tzarah isn't so great, and that he can be saved with his own strength, then he won't receive this extent of Hashgachah from above, and he won't have salvation. [This is because he isn't entirely reliant on Hashem]... Chazal say אדם של מזונותיו קשין סוף ים כקריעת, a person's parnassah is as difficult as kriyas yam Suf. This means that when one thinks אדם של מזונותיו, "the food of man", is אדם של, up to the work of man, it will be סוף ים כקריעת קשין, difficult for him to attain parnassah like the miracle of Kriyas Yam Suf. As Rebbe Elimelech teaches, lacking bitachon prevents parnassah. But if a person believes that parnassah is from Hashem, all he must do is a small hishtadlus, and he will receive parnassah easily. He won't need to work hard to attain it.
"I heard my father zy'a once say, 'May Hashem help me that I should be able to bestow on people parnassah as easily as I can be עקרות פוקד, to give blessings and salvations for the barren to bear children.' I tell the truth; I never understood my father's holy intention. עקרות פוקד, for the barren to bear children, requires a miracle beyond nature. How could this miracle be easier for him to perform than to bestow parnassah to people? But based upon the above, it is understood. When one is barren, he doesn't rely on any doctor or on any person. He knows with certainty and without doubt that there is no one upon whom he can rely other than Hashem alone. Therefore, he prays a lot, and he says chapters of Tehillim with tears, before the One who can help him bear children. It is easier to bring such a person salvation. However, if he trusts even a little bit in doctors, it won't be easy to bring him salvation.
"However, when it comes to parnassah, a person works many hours, inside the house and outside in the field. He trusts in his abilities and wisdom, and he thinks that if he works a few more hours, he will have more parnassah. To such a person, it is very hard to give a brachah and a salvation for parnassah. For this person, parnassah is as hard to attain as kriyas Yam Suf, and even harder than that."
At the Yam Suf, before the sea split, the nation sought solutions, what to do to get rid of their pursuers, the Mitzrim. At that time, they trusted in their abilities. But then Nachshon ben Aminodov jumped into the sea, and the sea reached his neck, and he knew that only Hashem could save him. When the bitachon was completely in Hashem, that is the moment they were saved.
