OCMock automatically use mocked instance in code under test?

I am new in OCMock.

I use dispatch_once() created a singleton class MyManager :

@implementation MyManager

+ (id)sharedInstance {
    static MyManager *sharedMyManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedMyManager = [[self alloc] init];
    });
    return sharedMyManager;
}

I have a method in School class which uses the above singleton:

@implementation School
...
- (void) createLecture {
  MyManager *mgr = [MyManager sharedInstance];
  [mgr checkLectures];
  ...
}
@end

Now, I want to unit test this method, I use a partial mock of MyManager:

- (void) testCreateLecture {
  // create a partially mocked instance of MyManager
  id partialMockMgr = [OCMockObject partialMockForObject:[MyManager sharedInstance]];

  // run method to test
  [schoolToTest createLecture];
  ...
}

I noticed that with OCMock, after I created the partial mock of my singleton MyManager instance, when run my method under test, it automatically use the partially mocked instance.

This is a bit weird to me, since in my test case above, I only created the partial mock of MyManager instance without injecting it to MyManager class,

how does OCMock automatically force the code under test use this mocked instance when [MyManager sharedInstance] is called in the code under test ? Could someone explain to me this?