UNO R4 WiFi SPI Master

Official Hardware Classic UNO R4 WiFi
1

Hello everyone,
I tried to using SPI of UNO R4 WiFi.e.g.:

#include "SPI.h"
#define CS_PIN 10
#define SPI_CLOCK 100000

void send(void) {
  uint8_t ret;
  SPI.beginTransaction(SPISettings(SPI_CLOCK, MSBFIRST, SPI_MODE0));
  digitalWrite(CS_PIN, LOW);

  ret = SPI.transfer(0x78);

  digitalWrite(CS_PIN, HIGH);
  SPI.endTransaction();
}

void setup() {
  Serial.begin(9600);
  pinMode(CS_PIN, OUTPUT);
  digitalWrite(CS_PIN, HIGH);
  SPI.begin();
  send();
}
void loop() {
}

The SPI is in master and mode0,here is a screencam of the oscilloscope:

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C1 is SPI_CS;
C2 is SPI_CK;
C3 is SPI_DATA;
why the cs pin is polled high before the data is sent over？

3

Thank you