Facts forming parts of the same transaction = The doctrine of Res Gestae.
The literal meaning of Res Gestae is - things done
Res gestae is a doctrine developed under English law and can be understood more clearly from examples and case laws.
Ex-A was running with a knife towards B and B was shouting save me and C heard it from a distance, So, since c heard it, he can be called a witness for the crime. Thus, whatever C heard will come under Res gestae.

Other relevant illustrations are:

• Victim's or injured person's cry for help.
• Sound of a shot of a bullet.
• Witness's cry while seeing a crime happen.
• Gestures, etc of a person in need of help.

Therefore, whatever is happening and whatever is said or heard, everything will come under RES GESTAE. The doctrine of Res Gestae took time to evolve further. Earlier, it was stated that Res gestae only means whatever is done/heard or said during the crime. After the crime is committed or completed, then whatever is said or heard will not be counted under res gestae. Let us understand this through case law. Case law – R v/s Bedding Field (1695) 6 Skin 402 A couple were living together happily. After a certain time, things got worse and one day during a heated argument the guy tried to cut the girl's neck. Escaping that, the girl tried to run away to save herself; while she was running, she saw her aunt and confronted her about what her boyfriend had done. The statement of the girl given to her aunt was not accepted as res gestae since the statement was given after the completion of the crime. Hence, the court concluded that things said or heard after the completion of the offence will not amount to res gestae. However, this perspective of Res Gestae changed after the case of Ratten vs Queen (1971) 3 WLR 930.

Other than this, there are a few similar cases such as R v/s Christie. In this case, a child reported sexual harassment and inappropriate touching to her mom. The court said that such a statement would not amount to Res Gestae since it was given after the completion of an offence. Ratten v/s Queen (1971) 3 WLR 930

A telephone operator of the police department got a call from a woman; the woman was very panicked and she demanded that she needed to talk to the police. The operator asked for her address, and as soon as she provided her address, the call was disconnected. Under suspicion, the police officers were sent to her address and she was found dead because she was shot by a gun. The husband said to the police – I was cleaning my guns and by mistake, the trigger got pulled and the bullet hit her.

The questions before the court were - was it the woman's apprehension that she might be killed or was it a coincidence? And if it is a coincidence, then can you call the police beforehand? The court re-examined the doctrine of res gestae and concluded that Yes, in this case, the words of the woman will form a part of res gestae and it will also include things which took place immediately after the offence. This leads us to the case: R v/s Foster (1834) 6 C & C 325; 172 ER 1261.

Here a truck driver was driving the truck at a very fast speed and then it rammed into a car. The victim's wife was standing at the corner of the road; she didn't witness the accident but saw the speedy truck. After the accident, she ran towards her husband and her husband told her that this particular truck rammed into my car. So, the question is will the husband's statement come under Res Gestae since it was given after the completion of the offence? and since his wife was not an eye-witness of the accident?

Court ruled that, Yes this statement will form a part under res gestae because this was said immediately after the offence.

So, this brought us to the evolved part of the doctrine and concluded that whatever is said or heard or happened immediately after the offence will also form a part of res gestae.
Immediately means when the person doesn't even have the time to think, it should be a sudden reaction. And hence Res gestae will also be called an exception to the hearsay rule of evidence.

Conclusion:
Section 4 of Bharatiya Sakhshya Adhiniyam deals with the doctrine of res gestae under the title – "Facts forming part of the same transaction" Res Gestae is also an exception to the hearsay rule of evidence.

Res gestae is important because it helps us understand how the judicial system tries to accumulate and examine relevant information available during the happening of an offence. It helps us understand the evidence available during the commitment of an offence on a deeper level.