Motion streak code using SKShapeNode that didn't make it into the book iOS Games by Tutorials (raywenderlich.com).

MotionStreak.h

#import <SpriteKit/SpriteKit.h>

@interface MotionStreak : SKShapeNode

- (void)addPosition:(CGPoint)position;
- (void)updateStreak;

@end

MotionStreak.m

#import "MotionStreak.h"
#import "SKTUtils.h"

static const int MaxPoints = 20;
static const CGFloat Thickness = 8.0f;

@implementation MotionStreak
{
	UIBezierPath *_path;
	CGPoint _points[MaxPoints];
	CGFloat _angles[MaxPoints];
	CGPoint _previousPoint;
	int _count;
	int _index;
}

- (instancetype)init
{
	if ((self = [super init]))
	{
		_path = [UIBezierPath bezierPath];

		self.lineWidth = 0.0f;
		self.fillColor = SKColorWithRGBA(255, 255, 255, 64);
		self.antialiased = NO;

		// NOTE: This is necessary because of an issue with Sprite Kit.
		// If the ShapeNode does not cover the entire playing area when it
		// is created, then it gets hidden in the areas it doesn't cover,
		// even if you add those later. I consider this a bug.
		[_path moveToPoint:CGPointZero];
		[_path addLineToPoint:CGPointMake(1000.0f, 1000.0f)];
		[_path closePath];
		self.path = _path.CGPath;
	}
	return self;
}

- (void)addPosition:(CGPoint)position
{
	// Ignore the new point if it is too close to the prevous point.
	if (CGPointDistance(_previousPoint, position) < 2.0f)
		return;

	_points[_index] = position;
	if (_count < MaxPoints)
		_count++;

	CGFloat angle = CGPointToAngle(CGPointSubtract(position, _previousPoint));
	_angles[_index] = angle + M_PI_2;

	_previousPoint = position;

	_index++;
	if (_index == MaxPoints)
		_index = 0;
}

- (void)updateStreak
{
	[_path removeAllPoints];

	if (_count > 1)
	{
		int i = _index % _count;
		CGPoint point = _points[i];
		CGFloat taper = 1.0f / _count;
		CGFloat s = sinf(_angles[i]) * taper * Thickness;
		CGFloat c = cosf(_angles[i]) * taper * Thickness;
		[_path moveToPoint:CGPointMake(point.x + c, point.y + s)];
		[_path addLineToPoint:CGPointMake(point.x - c, point.y - s)];

		for (int t = 1; t < _count; ++t)
		{
			int i = (_index + t) % _count;
			point = _points[i];
			taper = (t + 1) / (float)_count;
			s = sinf(_angles[i]) * taper * Thickness;
			c = cosf(_angles[i]) * taper * Thickness;
			[_path addLineToPoint:CGPointMake(point.x - c, point.y - s)];
		}

		for (int t = _count - 1; t >= 1; --t)
		{
			int i = (_index + t) % _count;
			point = _points[i];
			taper = (t + 1) / (float)_count;
			s = sinf(_angles[i]) * taper * Thickness;
			c = cosf(_angles[i]) * taper * Thickness;
			[_path addLineToPoint:CGPointMake(point.x + c, point.y + s)];
		}

		[_path closePath];

		self.path = _path.CGPath;
	}
}

@end

SKTUtils.h

You can find SKTUtils.h at http://github.com/raywenderlich/SKTUtils

Yeap, this is brilliant. It only took 2 mins to set-up and implement. Many thanks!